How to cope with so many changes... We are all so different...?

A bit more than a year go, I met a lovely girl, Mel. I am a gay man and she is a straight female. We became best friends and moved in together shortly after. We spent nights watching movies, talking, taking dance lessons, mocking the homeless on the streets, and she would talk about her life to me openly. As a virgin and a Christian, I didn't have much to say, but I felt like she trusted me... I then confronted her after I learnt she had had with her middle age aunt, and she attacked me. She moved out shortly after and we didn't speak for a while. Recently, we've become friends again and she moved back in with me but... She's, I'm afraid, nothing more than a parody of her old self. She's become vulgar, rude, unpredicatble, mean, and a nerd, who spends days studying for college projects. How can I deal with the fact that nothing will ever be like it used to be?