Emotion are weird. Sometimes, we love and care for someone so much and we want to be able to forgive them, we want to move forward, and we want to have a happy ending.

But sometimes, we're hurt so deeply, the actions have caused so much insecurity, and we can never regain the trust we once had.

First, this is not necessarily yours alone to "fix". There has to be consistent communication on how to heal the relationship. There also has to be a very fine balance. You can't forever punish your partner by controlling her actions (not saying that you are). Trust has to be regained by allowing her the freedom to show you she's truly changed and in it for the long haul. But even then, not all relationships can be repaired.

If you want to try, you can try individual and/or couple's therapy. If that, for some reason, is not an option, perhaps trying self-help relationship books, Love Languages, etc. It should be something you are both committed to, or else, it won't work.

Otherwise, if you cannot regain the trust you once had, it's okay to move on. It doesn't mean you don't love her, it doesn't mean you're giving up on her or the progress she's made; it just means you need to give yourself the opportunity to be happy without the resentment caused previously and without having to deal with the fears and insecurities she caused.