Hi pyrogen, sorry you feel so lonely. Your post resonated with me as I have been in a long term (8 years) relationship and have felt like you described for the last few years. Actually this morning my girlfriend and I finally had a big talk about it and it looks like we might actually be going to break up. It's weird, as much as I have dreamed of being able to indulge myself in things like, going to the gym as long as I want, watching movies all day or playing guitar whenever I want, etc. now that it looks like that is going to be a reality I am really scared and feeling lonely. I, like you, have kind of lost all my friends due to this relationship and the thought of living on my own again makes me really lonely and scared...but also underneath that feeling of loneliness is a feeling of excitment. I know that I am going to be sad and lonely for a while (maybe quite a while) but I think it will eventually turn into a good situation. But you know what? If she came home tonight and said she wanted to stay together, I would probably say "Yes! Awesome!" But, deep down I think I would know that we should probably breakup...but breaking up is hard, and sad...and being lonely sucks!!
Anyway...I hope my ramblings helped you in some small way.