I'm ever so much the loner who never makes friends, not because I don't want to, but because my attitude towards friendship is different to most people my own age, I have lots of friends who are much older than me, who have different health problems, mostly mental health but from different backgrounds, considering I'm female and most of my friends are much older men, and not female.
I've had the belief for many years now that most females of my own age are very immature, and like to follow each other like sheep, they love to watch movies, go clothes shopping, or go out on the town drinking on girls nights out, and have a lack of understanding or tolerance of any form of illness that makes you even remotely different.
Like you I have experienced people leaving me on my own, but mostly find much older men more loyal and less demanding than most friends, I do have some female friends, but I am very selective, most are the older female that have a mothering nature, there are a very select few around my own age, and although they are very good, supportive friends, I don't see them that often, but that doesn't matter as they are busy getting on with their lives, so when I do see them it's quality friendship time together
I've also had the lying friends, who believe me are not really friends, but when you leave school you will make some real friends who will be there no matter what, you'll also likely have some old school friends who although stay in touch you won't see a lot, that's pretty normal, you need to make friends and be around people who make you feel positive, and until then I'd suggest you just concentrate on yourself and what matters to you, don't waste time like I did trying to concentrate on others, and what they think of you, I did this for far too long, leaving myself feeling unhappy for far longer than I should have been.
*hugs* hope you manage to find some real good true friends that help you feel good about yourself
