Okay- so we all feel isolated at times (it just so happens your time is slightly longer than others), but seriously, really make an effort- you can't change things over night. I'd suggest every day going out for a walk in a populated area and then make a point of smiling at people and making eye contact- if you think it's a bit random, then go into shops and make eye contact and conversation with the shop assistants.
Go to the gym- you'll meet new people, it'll build your self confidence and get you fitter. Also try and get a job. If you don't look the part, you're considerably lowering your chances of getting one, so smarten up and be pro-active in looking for a job.
Try and join some kind of society, best bet is to re-join a college course where you can meet new people and join clubs and stuff there- that really is the best way to do all this. If you get down (which is likely, hardly anyone has a pain free life), then try to pick yourself up and start afresh. Be resilient and above all, be positive. You weren't born with some kind of permenant barrier which barred you from having friends- you put that 'barrier' there yourself. It's your job to remove it- and it'll be a slow process, but just keep making sure it is moving. I pretty much believe that there's hardly anything that you can't change pyschologically- you just have to realise your potential. Once you realise that you're valuable- other people will too. You have to open yourself up to other people otherwise they have no chance of knowing you.
It's all there already- you just have to discover it and make it happen!