Is my relationship going to hell?

I have been in a relationship with this girl for a while. I do love her and she said she loved me but I have my doubts. She Had a horrible past (3 years ago) that she doesnt want to share and she is very shy and doesnt say much as it is. I know we all have our secrets but she lets it controll her life. ever sence the incident she never talks to anybody but her best friend. I have been friend with her best friend sence 2nd grade and she doesnt even want to get into it. I want to know what is wrong so I can make her feel better and such. Every time i bring it up her eyes water and she stops taliking for me for a few hours. All I know is that it had to do with one of my bitchy Ex-Girlfriends and that whatever happened.....****** with her to the point of suicidal thoughts. I dont want any secrets in our relationship....not if it bothers her that much. I know this is all very confusing but can anybody Help by giving me a clue of what might have happened or any tips....PLEASE!?!