Don't really like this sort of thing because every person is different and simplistic solutions are never really the answer. It's always more complicated than just, "Well, find something you enjoy and do it."
So what I would suggest is that--if you don't already have a job, get one. But get one where you have to be active. Whether that's in a store or outside or whatever is your prerogative but I wouldn't recommend a job where you sit in a chair all day.
I suggest that because, first--having a job means that there is an element of structure to your life and that's a good first step. Secondly, having a job where you have to move around helps keep you from being lethargic. It's also good exercise. And while I love a brain teaser or a job that requires me to think--sometimes it's surprising how nice it is just to let your brain take a break and move on autopilot.
Doing this was very helpful for me at one point in my life. And also, looking after your health. Cut down on the booze if you're really into alcohol. And no, girlfriends don't solve problems. Compounding emotional instability with the stress of a relationship can make it much, much worse. If you already have a girlfriend--keep communication open with her.
If you can't find a job right now because of the shitty economy, maybe consider going back to school? That also can lend you some structure, connect you with resources, professors and open a door or two to something you might not have considered before. I don't know how the university system works in the UK but in America--a lot of schools have study abroad programs (although the concept of being abroad might be different in England because in America it's really exciting, as you guys are all the way across the ocean, whereas, for you--it's all just next door)
I found that making small changes like this can help you make big improvements over a period of time. Getting a job or going to school or changing health habits are all little ways to make you feel like you're in control. And I know that can be a difficult thing sometimes--when you're low, you feel like it's you against the world, when you're high--you feel like you can barely hang onto yourself. So when you start feeling like you're in control of certain elements of your life--that can give you confidence to start taking bigger steps.
This is all just things I've seen. Good luck.