The dilemma I’m concerned that being in a relationship with a (wonderful) boy has brought out the worst in me. He is selfless and endlessly giving, but I find myself transformed into a far uglier and less considerate version of myself when we are together. I am impatient and hyper-emotional, crying and feeling angry far more when I’m with him than when I am with friends or alone. We met two years ago when we were both 14 and a lot of change was happening in my life. My parents had unexpectedly had a second child the year before, and shortly after meeting him I went on hormonal contraception. I’m worried I have attached my unstable emotional state from that time to him, forcing him to be a sponge for my petty adolescent worries. I’m also worried that my mood swings and impatient behaviour are hurting him and that he is too kind to say. Though we have lots of fun and he is a fantastic companion, I tend to focus on his downsides and think a lot about us breaking up. Should I leave him, to protect him and me from the nasty version of myself that I put forward in his company? Or is that just a lazy way of getting out of my bad behaviour?

Mariella replies In the nicest possible way, he’s irrelevant. He sounds like a decent guy and if you feel like continuing the relationship, you should. On the other hand, if you genuinely feel happier when he’s not around you might want to try a spell apart. Either way the most important thing to focus on at the moment isn’t him, but your own state of mind. He’s not inspiring this behaviour in you but it certainly sounds like he’s letting you get away with it. That isn’t always the kindest thing to do.

You’re coming to the closing stage of one of the two most turbulent hormonal stages in our female lifespan and it’s a time when rationality famously gets overridden by far stronger forces. As humans we appear to be programmed to push boundaries to breaking point, which is positive when it comes to our ambitions, and often less so when it involves our psychological impulses.

The people we love also carry the burden of our bad moods

I’m so impressed by the letter you’ve written. It displays an emotional maturity and understanding way beyond your years. You’ve articulated very clearly how you’re feeling and even tried to get to the bottom of what it might be that’s prompting you to behave in this fashion. That’s the absolute best approach you can take. Your letter is blessedly lacking in self-pity, which is not something I get to say often. Instead, I think you might actually be giving yourself too hard a time.

The people we choose to share our lives with get to bask in the warm glow of our love, but they are also the ones who tend to bear the burden of our moods and miseries. It’s far easier to hang out with a friend, distract yourself with their company and then run home and scream at your partner. So, in essence, the way you are behaving at the moment is pretty normal, if perhaps exacerbated by circumstances beyond your control.

We know there are contributing factors here and that’s a good start. Your parents having a surprise second child when you are particularly vulnerable will certainly have contributed and, while you might be uncomfortable about punishing the baby, your boyfriend is a delightfully easy surrogate target. Whenever you feel frustrated and angry and want to hit out, there he is, stepping conveniently into the firing line. Good people don’t necessarily make perfect partners and it is possible that your frustration and anger towards this boy is because you’re not compatible. On the other hand, you may simply be offloading on to the nearest shoulders broad enough to take the weight. If that’s the case, you need to reboot your behaviour or you’ll lose a decent bloke.

Your mood swings are partly hormonal and I’d definitely recommend a visit to your GP to discuss trying a different contraceptive pill. For me there was only one, of all the many brands available, that didn’t make me feel peculiar. Dealing with that is a good practical start and may lead to you feeling better generally.

Finally, there’s this upstart sibling suddenly in residence. But it’s worth remembering what a gift this may be. This newcomer won’t reduce your standing because, as every parent knows, love isn’t finite but infinitely expanding.

You are young, and it may not be comforting news, but you and your boyfriend probably won’t be together for ever. That’s not a reason to split up now, but it is a reason not to let the current situation persist if it’s making you unhappy. You’re not to blame, or in the wrong. You are to be commended for your self-awareness and desire to be fair. It may just be that you need some space and time to understand your own emotions before you become too embroiled with someone else’s.

You are well equipped to overcome your dilemma, but be gentler on yourself. Think carefully about what would make you happier and then act on those instincts, rather than trying to guess what’s best for someone else.

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