If you love her trust her, she was brave enough to tell you her past she probably had own reasons of why she did that, you cant jump into conclusions if you don't know why she did that don't assume too quick everyone has reasons of why they do stuff like that ask her :) you shouldn't judge her by her wrong doings on the past she might change for you :) stop living on the past, live for the present and the future a blessing is on you guys way :) good-luck
All this "once a cheater all a cheater" is bull****. I know people who cheated on somebody but then they got married to the person they cheate with and never cheated again. Personally, because she told you, it means she's getting serious and wants all secrets and problems out of the way so she feels good about this and doesn't have to hide. She is trustworthy. Why else did you propose to her?