Man its crazy how many people (including myself) can relate to you flappy... I have a hard time trusting my g/f 100% as well. Although...she has done a few things behind my back, insignificant they may be. (I've caught her smoking behind my back on like 4 occassions). The reason I think I can't trust her 100% is because I know what she was like b4 me...her friends, her lifestyle, really bad things I've heard and some were found to be true....but in reality. She has changed after she met me, she's not the same and I really have no reason not to trust her.
We've gotten in many big fights over the trust issue as well as my insecurity issue, and I always make it a point to talk to her about it. And I've been trying really hard to change my ways...it is hard to change the way you are and sometimes just let her do wahtever she wants and do it her own way, but once you do it for the first time, it gets easier and easier afterwards.
Chances are you're g/f does really love you, four years is a long time and you shouldn't let it go to waste. Give her a few days, then really try to get to her and talk to her. Like someone said...talk to her like you're talking to us. Tell her you realize you have some trust and insecurity issues, but you want to change for her, and because you love her and want to be with her. And you'll pay any price to be with her....(if thats the way you truelly feel.)