Good post mate.
In terms of girls - I went years without a girlfriend simply because there wasn't anyone I really connected with. Then a few years ago I met a South African girl who I really liked - went out for a month or so - and then she left for Perth for a new job and within a week or so was in a relationship with her ex from a few years ago. That took a week of constant sighing (also during the last week of my undergraduate degree and with 10,000 words in essays to write) . Then November 2013 I met a German girl, knew she was going away to Germany in six months, but fell in love with her in a relationship which was kind of premeditated on knowing we only had 6 months. Which in hind-site probably wasn't healthy. So we spent every day together - but drew some weird boundaries so not to get too attached, and that probably ended up being one of the most difficult things to cope with. Going to Germany later this year to hopefully see her and we're still in regular contact (although not ending it obviously doesn't make it easy to put it out of my mind), but I've spent half a year feeling incredibly down about how unfair life can seem. My advice on girls (which I ignore frequently) is that you know the girl is right for you if she genuinely makes you feel happy and secure. If that isn't there, no matter how incredibly infatuated with them you are, it's just not going to be a happy relationship. If your feeling jealous, or suspicious or insecure - there is probably a reason for it. And it's probably just as much about how you are feeling about you.
In terms of depression, like it seems a lot of people on here I've been in a similar dark road. I have a few times, much like others through my early years of high school and much more recently, when you think "What's the point? Things aren't going to get better. It would probably just be easier if it was over". What has helped me is actually writing down how I've felt during some of these points. And when they pass you can read how you felt. Sometimes I read it and I think it was written by a completely different person, someone who just seemed to lose hope. I remember in 7th form I wrote a letter to myself as part of a class project to read for when we left in 7th form. The contents were generally based around what you had hoped to achieve by the time you finished. My letter read something along the lines of "I hope to have achieved not being such a fat loser, because my life sucks" - in more detail. I was genuinely astonished as I didn't feel that way at all about myself by 7th form. I wrote a letter last year which I haven't had enough time pass to open yet, but overall when I think back to how astonished I was in 7th form - it is encouraging that the way I have been feeling probably won't be my reality in five years time.
Other than just time passing and the cloud lifting, having mates who can remind you that there is a reason that you're their friend, is worth its weight in gold. It's easy to think that you just don't have anything to offer - but the reality is that if there is anyone out there who likes you, there is probably a reason why. Maybe you're funny, or you go the extra distance to help people, or your genuinely a good friend. You only need one thing going for you, and you probably have a lot more than that. Just remember those things - and if you can't think of any ask a good mate.