"I never saw myself as a captain," Sam Warburton says with distinctive honesty. Stepping away from his triumphant role as the leader of the Lions in Australia, and preparing for another autumn series with Wales, Warburton confronts the demons and pressures that often accompany international captaincy in unflinching detail.

He is a fine openside flanker and, having turned 25 a few weeks ago, on the path towards sealing a monumental career with Wales and the Lions. Warburton is also composed and amiable, and a treat to interview, but he prefers being quiet and solitary on match days rather than a rabble-rousing captain. He even confesses to almost breaking down on the field towards the end of a Test against the world champions New Zealand.

Leadership was foisted on him at a young age because, invariably, he was his team's best player. Warburton, however, nods emphatically at a suggestion that being singled out as captain went against his instinctive introspection. "Definitely. When I was growing up my motivation was never to be captain. It was to achieve things like being on a winning Lions tour and World Cups and grand slams.

"When I was captaining all my age group sides I thought it would be good for my rugby CV and help me get a contract but I always found it uncomfortable. I probably found it uncomfortable in my first 10 to 15 games as Wales's captain but now I've done it 20 times, and with the Lions, I'm completely comfortable. It took two to three years to reach that position of comfort because it's different captaining your country. You're under the microscope constantly. The toughest thing about [international] captaincy is that if you don't give an eight out of 10 performance then you get criticised and it's suggested you could be dropped. Emotionally, that's difficult."

Warburton picks out the lowest moment of his tenure as captain. "I remember it as if it was yesterday," he says. "I was sat on a plane, going out to play Leinster, and I thought, 'Oh, I'd love to be that guy instead of me …' He was the guy on the runway doing this [Warburton holds up the imaginary beacons of an aircraft marshall directing planes to and from the gate], waving his hands. I thought how much I'd love to swap places and have a job with no pressure. I texted my mum and said that before a game against New Zealand in an autumn international. There was so much pressure going into that game because people had been criticising me harshly. That whole week's buildup was one of the most difficult I'd ever been through.

"I played and by about the 65th minute I knew I'd had a really good game. I would have given myself a nine out of 10. And so then, in the 65th minute, I nearly broke down on the pitch because of the relief. The overwhelming feeling was relief that people could get off my back for a while."

Warburton shakes his head at his own jumbled emotions. "You go through quite a bit and you have moments when you feel it's not worth it …"

He is such a warm and generally positive man that his face soon brightens. "But then you have moments on a winning Lions tour that completely outweigh any negative thoughts."

Warburton has just written an intriguing book which hints at the complexity of his experiences. The flashes of insight into the bruising vagaries are made all the more immediate by the fact that earlier this year he captained a Wales team who had suffered a record eighth successive defeat. Injury then intervened and, even when he regained full fitness, Warburton could not reclaim his place against Italy in February. At that stage of the season, with England unbeaten in the Six Nations and having gained an exhilarating victory over the All Blacks last November, Chris Robshaw had edged ahead of Warburton in the media-driven race, alongside Brian O'Driscoll, to captain the Lions. In the end, after Wales dismantled England in a tournament-winning finale, Robshaw did not even make the Lions squad.

But the testing nature of last winter lingers for Warburton. "The most frustrating thing for me was that I didn't feel I was playing badly. I wasn't missing tackles. I was actually quite happy with my form. But it makes a great story – and once you accept the captaincy you're going to be put in these situations."

Warburton admits that the invasive scrutiny led to a loss of self-belief. "That really happened. I've always had the utmost belief in my abilities. Ever since I was a kid I've believed I was better than anyone I was playing against. That's not me being arrogant. It's the way you have to think in sport. That self-belief went for a while and Andy McCann [his sports psychologist] had to get me back on track."

Recalled to the team for Wales' penultimate Six Nations match against Scotland, Warburton thrived without the pressure of captaincy. He produced a man of the match performance and then asked the interim coach, Rob Howley, not to offer him his usual leadership role when the stand-in captain, Ryan Jones, was injured. Warburton also played at No6 because of Justin Tipuric's blistering form at openside – and England were humiliated.

Warburton was touched that Robshaw, despite his disappointment, sent him a supportive text after his captaincy of the Lions was announced. "It was really nice of him. I see how much attention Chris gets as England captain. He's always under the microscope so I understand what he's going through. But he's a great player and, hopefully, I'll have the chance to play with Chris one day."

Warren Gatland, Warburton's coach with Wales and the Lions, was condemned for his decision to omit O'Driscoll from the 22-man squad for the third Test against Australia. O'Driscoll had played solidly in the first two Tests and his Lions and leadership credentials made him the obvious candidate to replace the injured Warburton as captain.

"Obviously, it was a pretty controversial event but I supported Warren by saying you could pick either Brian or Jonathan Davis and they'd do a fantastic job. He took a monumental amount of stick but we won by that huge [41-16] margin. It was probably the most dominant performance against Australia by a northern hemisphere side – ever – so Warren should take credit. He had been under immense pressure but he didn't gloat. He was very humble in victory."

Did Warburton talk to O'Driscoll after he had been dropped? "No. I didn't want to seem patronising. You just try to act normally. He's been through a lot and so if anyone could handle that it would be Brian – and he handled it great."

After he had been injured during the second Test defeat, Warburton spent part of that evening with his parents and his fiancee, Rachel. Just before he retreated to bed his mother told him that one of his beloved dogs, Gus, had died back home. "As four-hour periods go, it was pretty bad. I tore my hamstring, we lost the second Test and I lost my dog."

He laughs ruefully. "My fiancee has only seen me cry twice since we've been together – for seven years –and that was one of the times when I just broke down."

Rachel and Sam have vowed to replace Gus, named after Gus Poyet, with a new Spurs-inspired mutt. "We're going to get another dog and you're the first to know his name," Warburton says as he quips about this rare exclusive. "We're going to call him Ledley. Ledley King's a real hero of mine."

Before then Warburton confronts another testing autumn of four internationals – bookended by matches against South Africa, this Saturday, and Australia on 30 November. "I don't watch any rugby normally but South Africa against New Zealand [in last month's titanic Rugby Championship decider] just grabbed me. It was awesome and one of the best games of international rugby I've ever seen.

"We've got South Africa first up and it would've been better to play a team lower down the rankings as we've always started slower than we would have liked in past autumn and Six Nations campaigns. So we're going to have to crank up the intensity in training and hit the ground running against South Africa.

"But Australia is the most eagerly-awaited match for the Welsh public because we've had some desperately close matches and lost by one or two points. It's probably the eighth time I've played the Aussies in two years. We've broken the pattern of defeats with the Lions and the motivation will be really high going into that match."

Warburton is only three games into his comeback from injury but his place as captain and at No7 for Wales seems assured. "Justin has been playing really well," he says of Tipuric, "and if I was asked to be part of that combination again [with Warburton at No6] I'd do it because I enjoyed it. But I don't think it would work against South Africa. I think you need a battering ram at six when you're playing such a physical side."

His load is lightened at the Cardiff Blues by a joint decision for him to just be a player. "The coach, Phil Davies, agreed. It's hard to do captaincy all year round. It would end up pulling you down. Phil knows I'm happy to captain the Blues whenever they need me but it's quite nice that I can be selfish and just concentrate on myself."

Warburton was one of the key performers in the Blues' recovery from their shattering loss in the Heineken Cup against Exeter – who led 36-3 at half-time. "It was like panic stations – real SOS stuff," Warburton says of the following week's salvage operation which resulted in victory over the European champions, Toulon, in their next game.

"We did lots of team building that week. It was unusual circumstances when you've been part of a victorious team, in the Lions, which is so rare, and to follow that by going 30 points down to Exeter. But Toulon was the best club game I've been involved in by a long shot. It was an incredible turnaround in six days. A lot of the players are quite young and, if anything, it was a lack of confidence and them not knowing how good they could be. Toulon have all these stars but you realise that they're just human and everyone can be put under pressure. It was a valuable lesson for our younger guys."

Warburton sounds as mature as he is resolved after all his personal battles. He has discovered some kind of comfort, at last, in leadership. And it is striking how clear he is when asked if he aspires to emulate his achievement in Australia on the next, far more unforgiving, tour with the Lions – in New Zealand in 2017. "Absolutely. I have that hunger now – big time. People say it's wrong to look forward to things but I am now. It's a long way away but that's definitely my long-term goal. To win in New Zealand, with the Lions, would be incredible."

Sam Warburton's Lions Triumphant: The Captain's Story is published by Simon & Schuster