The dilemma I fell in love with a colleague who had a long-term girlfriend. Eight months after meeting him he told me he had been infatuated with me since I started, and we kissed. A few weeks later we slept together. He has told me his girlfriend is now his ex and I whole-heartedly believe him though he still lives with her, in the spare room. I spent Christmas with his family, who are understanding and keen for him to leave her. She drove a wedge between them, but his family like and accept me, even giving me small gifts.

Four months into our whirlwind romance and his mental health has snapped and he’s started seeing me much less frequently. He broke down over the phone (a few days ago) and told me he now wants to end it with me, as he can’t cope with the stress. They have a mortgage and a dog together. He refuses to give up on either of them and asked me to give him time and space. We were due to be in France for Valentine’s Day and now I am stuck at home alone – we are due to go to two weddings together this spring. I don’t want to give up on him.

Mariella replies It’s not your choice. Just look at the knots you’re tying yourself up in trying to work out an acceptable narrative for his unacceptable behaviour. You say he’s suffering a mental health issue, but I’m not sure indecision qualifies as an actual condition. Nor is emotional cowardice a symptom of anything but itself. There are few qualities less attractive and this man of yours seems to be imbued with industrial quantities. If he’s been roped back into his former liaison via financial pressure and affection for his dog, it still doesn’t shine a favourable light on his actions.

Regular readers will be aware of my near 19th-century reservations about letting your emotional impulses rule your head. Surely we can all agree that when you’re being overlooked for a pooch it’s time to get real. This isn’t the stuff of great love stories, but of ill-conceived dalliances, self-deceit and our deep desire to be wanted, often at any cost.

‘Few qualities are less attractive than cowardice’

I’ve no means to gauge the depth of this colleague’s feeling for you, but there’s no harm in us imagining that his infatuation is genuine. The trouble is that doesn’t make him any less of a bad bet; in fact it increases my sense that this is a guy with little moral fibre and no ability to steer his own destiny – two qualities not at all appealing in a prospective partner.

Your letter is tellingly all about this man’s wants and needs: his struggle to break away from his past relationship, the ex-girlfriend his parents don’t like, the little gifts that suggest you might be a preferred candidate, misplaced worries about his mental state simply because he can’t walk out on his partner for this new liaison. I don’t even know him and I’m bored by how much everyone around him seems to be focused on his needs. What about you? What does he have to offer you? A job as mediator with his family, company in your apartment (as he won’t be buying his own in the near future), a needy dog that may or may not be your love rival. Come on, you’ve got to value yourself higher than that.

I don’t really care why he’s still with his girlfriend, or how much his family have held you to their bosom. Actions speak far louder than the support services he seems to be auditioning for and as far as I can gather, he’s been positively sloth-like in making your relationship anything more than a pleasant diversion from his own problems.

He’s also had an unprecedented amount of support from you. I’m puzzled as to what you were doing at his family home for Christmas when he is still living with someone else. It suggests that he’s surrounded by those who are complicit in letting him mismanage his affairs and conduct a double life.

How many more scenarios must I be sent before there’s a light-bulb moment where we all start to understand that if you’re in a bad relationship, starting a concurrent replacement is not the solution to your woes? I’m not the Virgin Mary and God knows I’ve been tempted at times – any sort of attention feels tantalising and unexpected when you’ve been married for 15 years – but I was taught that you honour your commitments, not try to dodge them until the moment you’re discovered.

As I said last week, if we’re looking for transparency and righteousness in all walks of life we need to tidy our own backyards first. But even in such a brave new world your man would come up wanting. Looking at your letter the only one of you four who I sense is happy with current arrangements is the dog.

My advice is to tell him you will give him neither space nor time but intend to carry on with your own life, rising above his inability to make choices and setting your sights on someone who knows what they want and is prepared to make the sacrifices. Treading water isn’t a skill to be celebrated, or a sensible way to live your life. And in this instance you’re both at it.

If you have a dilemma, send a brief email to mariella.frostrup@observer.co.uk. Follow her on Twitter @mariellaf1