For a long time you have managed to hide these emotions, but keeping them inside of you, is the worst thing you can do. What i don't see in your topic is why you feel this way. If you think that no one in the world understands you, try talking to people, and then you'll realise that a lot of people go through this. We all tend to hide things that really hurt us inside, in order to seem like nothing is wrong.
I also went through this. I told my closest friends about the problems that i had, and they reassured me that everything was gonna be okay. Sometimes all you need is to express yourself to someone who you're close to. I'm always here if you want to talk about it, just send me a message.
But I cant imagine doing anything else. Unless its a job, that way I'd have money to buy stuff and not have to spend ALL my time trying to find an unsatisfying fix to hold my nerves
You managed to hide your feelings at first because you had strength to do so. However, keeping up such an act for weeks or months gets more difficult as time goes on, and that's what's happened to you.
I suspect you're not going to like this, but I'm afraid the answer is to tell somebody. I know that hiding it all has worked for you until now, but it's not a tactic you can rely on any more. Now you need to face it, and if you want my advice on that, you'll do so as soon as possible. Sometimes dragging yourself out of a hole like this can take energy by itself, so you'll want as much left as possible for doing that.
I hear you when you say that most people will not believe you, so it's important to find support - real support. Full marks for not wanting to pressure your girlfriend with all of this, but it might be worth telling her how it's going.
Apart from that, there are specialist help forums online all over the place and you'll likely find practical help there, so Google whatever issues you have along with 'help forum' and you might well find an excellent mode of support.
Good luck.