How can I trust her again?

I lived with her for 4 years, I loved her and she loved me, she always told me that she never leaves me and whatever happens she is mine. I always asked her not to promise me any thing about future, because my ex-girlfriend betrayed me, and I just liked to live for now, but she always said ” I am different” and I believed her. after her father died, she got a kind of rich, she completely changed her attitude and simply she left me. before this death she was economically dependent on me and I paid for her expenses, she now tells me she likes to see me weekly and have sex with me! ( this broke my heart, do you feel what I say?) she doesn’t like to live with me any more, because she can have a better situation with her mom, she says she doesn’t like to have responsibility and she doesn’t like to share any thing with me , when I ask why, she says: I don’t know! I am confused.
I feel I am totally betrayed, I don’t talk to her any more and I can’t forgive myself, I shouldn’t have trusted her, but I can not understand how it really happens! how a person can change that much just because of money?! they have always some reasons to convinces themselves that they did the right thing. but this one even doesn’t bother herself to give me a reason.
I am too depressed, I cry every night, I haven’t cried for years. I even can’t tell my problem to my friends, I feel really embarrassed and misrable, I can’t sleep and I am not sure that I can continue working, I am a teacher and my depression soon will affect my job.
is there any way I can help myself to understand my situation and move on? am I really stupid and I should accept it?