Wendy Maltz's book "The Sexual Healing Journey" has a good list of ways to think about sex that are healthy. I'm not sure they are affirmations exactly, but they do help guide one's thinking in a good direction.
I think I have also found it true that an affirmation will work at moment when we are feeling vulnerable to beliefs about oneself that are holding us back. As scary as it can be, looking at what those beliefs are has been a good step towards finding solid affirmations.
Wishing you well at this crossroads,
Lose the drama; life is a poem.