Man, I know how you feel and all I can say is I am sorry. I am going through a very similar situation right now and I know how much it sucks. my gf of 2 years told me a month ago that she wanted space, was feeling weird, blah blah blah. Basically her space was hanging out with a guy she knew and I suspect they are 'involved' with each other now. I dotn think it will develop into anything serious but the fact of it is...they say one thing and they are really doing something else. You may of wanted to believe that she just needed space but as turns out there were other intentios. There is pretty much nothing you can do.
This is the second time I have been through this and both times I heard similar stories only to find that they are seeing somebody else. Its horrible to hear and you try to deny that it is happening sometimes, but the best thing to do is try and forget and move on. Maybe one day she will realize how stupid she has been and want to come back to you....maybe not. The only thing you should focus on is building yourself up to the point where you can think with your head and not your heart. Right now you are so overwhelmed with your emotions that its hard to make rational decisions. Cut contact with her and start to heal and you will see that your outlook will slowly begin to change.
Good luck man, keep me posted...I hope you feel better...