The dilemma I live with my partner of five years, who I adore, and his 17-year-old daughter. She doesn’t have many friends and never goes out, but she is a nice girl and has accepted me. She is sweet at times, but I get very wound up over little things and I dread her coming home. She gets moody and can be quite bolshy. She is close to her dad which is great, but it winds me up. For example, we went away for a couple of days and she was all over him, cuddling, putting her legs over his and always trying to be the centre of attention, which made me feel left out. A few times I have come back from work and found her lying on my side of the bed next to him chatting. I really don’t want it to affect my relationship with her dad, but she gets jealous when we show any sign of affection towards each other and that drives me mad. Am I being over the top?

Mariella replies She’s not the only one getting jealous, is she? But, importantly, she is the only child. You’ve known this girl since she was 12, so it’s disconcerting that you describe your relationship as being one of competing lovers, not a concerned adult or step-parent discussing behavioural issues in a kid you have a degree of responsibility for. The first thing you need to do is dispense with the delusion that you are locked in a battle for his affection. She is his daughter for heaven’s sake. His love for her would, and probably should, trump his love for you, so I really wouldn’t bring the level of debate down to a stark choice between the two of you.

There should be clear lines between your relationship with your partner and his with his daughter. You are engaged in a union between two adults, based on physical attraction, mutual compatibility and the enjoyment of each other’s company. Your partner and his daughter’s relationship is defined by primal parenting instincts which are pretty inescapable. They may fall out, but they can never “split up”.

So, what is it you’re fighting for? Finding her lounging on your bed is hardly an affront. A parent’s bed is a place of safety – it should be like a raft to board when the going gets tough. If you live with a child it’s not your bed, it’s a family lifeboat, which is one of many reasons why a healthy sex life can often become a challenge! What are you going to do, put a No Entry sign on the door?

Dispense with the delusion that you are locked in a battle

Direct confrontation and exclusion zones are not appropriate. What teenager, testing out her power, wouldn’t revel in a skirmish for her father’s affection, particularly with the odds loaded in her favour. It’s tantalising for someone her age, trying to understand her own power, to test herself by working out how to manipulate her dad. She’s practising how to step out into the world and to have her own relationships on the most important man in her life to date. You are meant to be showing her a good example of confident, mature womanhood, not quarrelling over the spotlight. It may sound harsh, but I suggest you summon some strength and dignity and stop pitching yourself as the opposition or at best you’ll look faintly ridiculous.

Giving credibility to such neurosis on paper makes me uncomfortable, so how confident do you feel about seriously claiming their closeness as a danger to your own? I’d be treading very warily if I were you, because if you force a choice you won’t be on the winning side, even in the unlikely and unfortunate event that he steps into your corner.

If you and this man intend to stay together, your relationship will be stress-tested many times with all kinds of outside pressures to negotiate. If you can’t cope with the most fundamental of these – helping nurture the daughter he already has – then I suggest you move on to less testing relationship ties. For an immature and probably insecure teenage girl, you’re creating a confrontation that’s all the more tantalising because you’re well up for playing the game.

I appreciate it’s not easy dealing with any teenager, whether they are your blood responsibility or not. If you hook up with someone who has a child, however, the deal is that you don’t behave like one. I’ve no doubt this girl knows how to trigger your insecurities, so a good place to start would be to stop being so easily provoked.

There is certainly an argument for creating boundaries so you can rub along more harmoniously, but stamping your foot only increases the stakes for any teenager. It sounds to me like you and your partner need to carve out more time as a couple while ensuring that when you’re at home the levels of affection between you aren’t dictated by his daughter.

That said, to be the only child stuck with two adult lovebirds is an unenviable position for her. Let this young lady grow up secure in the knowledge of her father’s unconditional love then step out into the world looking for a similarly functional and committed relationship for herself. Ultimately this girl will leave you both to your feathered nest, but until then it’s a communal living space for three people who care for each other – not a battle ground for supremacy.

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