Ok first, to explain what Vosur meant.
You are less than 16 years (don't know your exact age) through your life, you have an almost 80 year life, which means you are less than 20% through your life. In each moment of your life, each instant, there is an infinite amount of outcomes (though each outcome has a different probability of happening), having so many countless moments for so long with so many different variations, it's very difficult for you to lose your one chance of happiness, almost impossible in fact. So to say you have lost your one chance of happiness, when you have so much left, so much unexplored so many new circumstances you haven't even begun to think about, it's short sighted. You still have many more chances for happiness, you can still grasp it.
For your problems, I'm going to address my 2c on each of them, then I'm going to say what I think is important to do about "thinking" about the problems.
Friends: I think you can be a nice guy if you want to be, you just need to be confident, fear of rejection is a very hard thing to get rid of (if you have that fear) but if you feel that you need more friends, the best way is to hold your head up and go into a crowd with confidence and an upbeat attitude. If they try to bring you down, don't let them, if they try to make you look like a fool, laugh at yourself, don't get embarrassed, because if they see you being so strong to actually laugh at yourself and ignore all their bullying, they will be the ones looking like a fool.
Girl: First things first, you can't change the past, the past is over and done with, what can you do? Nothing. So, stop thinking about how what you have done is bad or wrong, because that won't achieve anything but make you feel worse. Instead, look to the future, because that's something you can change, look to the present as well because that's where you change the future. Think, how can I get around not acting like a dick to this girl? Can I be nice to her, maybe I should think of my Tiger stuffed animal when I'm talking to her, get a happy place and see if we have anything in common. I dunno if I'd ask her out straight away, but talk to her, get to know her more, be good friends with her and then tell her that you have feelings for her and tell her you were sorry for acting like a dick earlier. It's all ok, you can't change the past, but you CAN change the future and thinking about what you can DO is much more beneficial than thinking about what you have DONE
You can choose to think about all these issues very depressingly, as if things are going horribly wrong, or you can choose to attack the issues. IF the issues are changeable, change them! If they issues are not changeable, ignore them. It's an old saying, "If you can change a problem, what's the need in worrying? If you can't change a problem, what's the use in worrying?". Things will be bad and you can choose to let that destroy you, or you can choose to brush that off and you can push on. It's not as simple as a choice obviously, it's very hard to not feel sad, but it is something you should think about. I just think thinking in a positive mood can help a lot.
Hope it all gets well soon