*hug*
It is not as bad as you think. Everyone exaggerates stuff, I think. Personally, I don't think you should tell them; it's one of those perfectly natural things to do that nobody usually talks about, and it would suck if people were weird about it after you were brave enough to tell them. The lie isn't hurting anyone (except maybe yourself...). Don't think badly about yourself for lying. If you feel like it is a burden, maybe you could tell the first people that you broke up (amicably, not a dramatic, comfort-me break up) and put it behind you. You are not a prat that doesn't deserve any friends - seriously, people lie about much more serious things than details of their love lives (which aren't anyone elses business really). I think you have learnt from this experience that lying about this sort of stuff will make you feel worse, not better, in the future. And to be honest, there is nothing wrong with being single! You won't be single forever, I promise. I'm 21 and have not had any great romances in my life (nothing longer than a couple of months, how lame!). And telling people you are single sometimes makes them say exciting things like 'I should set you up on a date with my hot single friend'. Being embarrassed about being single is not a good way to meet guys!
I hope I said something of use here. I just think I know sort of how you feel because I told a complicated lie once and it made me feel bad for ages. But seriously, don't feel guilty or silly or anything - just move on from it. I doubt anyone suspects you lied, and nobody will be trying to catch you out.
Edit: I just realised that I suggested resolving your guilt of lying with another lie. I still think it's a good idea though, because it'll put it all in the past. Also, did you do Camp America this summer? Or is my memory rubbish? If so, how was it?