I think there is something wrong with meWatch
Please don't feel like you have to explain/defend your self harm to anyone, even your friends. People do this for a variety of reasons, or seemingly for no reason at all other than feeling low. The reasoning behind it (or lack thereof) doesn't make your experiences or your feelings any less valid and important. Having said that, I'm sure you're aware that it's not an ideal coping mechanism. The app 'Calm Harm' is free, confidential, and really good for helping you to resist urges to injure yourself. It has a lot of distraction techniques, which are great in the moment, but I would strongly suggest getting to the root cause of the issue and thinking about why it is that you're turning to self harm as a way of coping.
Obviously, I don't know the full reason behind why people are pressuring you to 'be less selfish', but if you're beating yourself up over this, it means you're a good person; people who are actually selfish wouldn't care about it. Living with such burdens is also really difficult, so it's normal for you to take this frustration out on others sometimes. Have you tried talking to your friends/parents/guardians etc. about why you might be acting this way? Even if you're not ready to tell them the extent to which you're having a hard time, it might help them understand if you simply explain that you're under a lot of pressure.
I know A-Levels are a lot of pressure and it can be so hard to cope with the workload, but you're doing so well and I'm so proud of you regardless of what other people are telling you. If you're struggling this much with it, though, have you considered taking an alternative route? You could do an apprenticeship or a BTEC, perhaps? Not everyone is suited to A-Levels and they aren't the only option, even if the education system and society deem them to be somehow more desirable than other qualifications (ew, capitalism). When we already feel so low, even the smallest things can hurt us more than normal, and that's okay. Have you thought about therapy or speaking to a counsellor at college?
In terms of friendships, it can be a challenge to keep up with people and tend to their emotional needs when your own are being neglected. I don't have the details of the situation, but I can tell you that it's probably not your fault. The same goes for romantic relationships. Try not to pressure yourself to conform to what society deems normal; many people have similar experiences to you. I'm 18 and I've also never been in a serious relationship, but that's just how it goes sometimes. It's not a reflection on you or your worth in the slightest.
If nobody has told you this today -- you matter, you're important, and I value you.