I don't think a little resentment is a bad thing as long as you aren't too attached to it. It helps keep all the " this time will be better and she really means it this time and now I can finally get the life I always wanted but never had with her " daydreams in check.
These are people who did things to hurt us and hurt us bad. In my case I feel it is fine to forgive but never forget.
Your friend,