I don't have a ton to say except that I'm going through something somewhat similar and maybe it will make you feel a little more..enlightened.. to know that you aren't the only one, seriously.
Moving along, ShaniFaye is right (isn't she always?
) You seem to have been very kind and supportive and if she wants to choose trash, let her. And *don't* get into the role of trying to fix/help her either as many kind people like yourself might tempted to be. I'd say the damage has been done and you shouldn't turn back.
Also, in regards to you feeling like you can't trust anyone right now and being worried about your future. As I said above I'm going through something very similar right now. I've been asking the same question and my main reassurance is that it's experiences like these that truely do make us stronger in that we'll be less inclined to date people capable of this trust gaining and breaking because we'll hopefully be more aware of it. I've got a friend who is so on top of his life right now, largely due to an experience I saw him go through when a girlfriend cheated on him. Of how ugly a situation it was, it remains bitter sweet as it truely has made him a stronger person and I see him virtually shine from the experience now as he's well past it and gaming kind, beautiful women out of his 24 year old mind