Captain_ usa do yourself a favour and finish it with her-the longer this goes on the more resentment it will cause between you two. She cant love you enough if she isnt willing to spend quaility time with you, if at 28 she still wants to act like an 18 year old and get drunk every weekend, I dont reckon she is mature enough to be in a relationship. Being in a relationship is about treating each other with equal measures of love and respect, and commitment and trust. Do you know what really happens when she is out and about? She may very well be getting off with blokes, but she wouldnt tell you, as what you dont know wont hurt.
I sincerely hope things work out, but to me she sounds like trouble, I would confront her about it in a nice, calm way, see if sahe is willing to compromise. If she isnt, then be a man and finish with her. Come on, you deserve better than to be tread on like a dorrmat, mate!
good luck :-)