I regret letting my wife sleep with her ex
Are you assuming she is going to go back with her ex?
I don't think that would happen since she is pretty loyal to me.
You shouldn't let your wife sleep with other people. It can lead to resentment and regret. You should both talk about this in counseling.
It's fine, I'm doing the most I can to show her how much I care about her.
Allowing your partner to sleep with other people is not a great idea as far as the relationship goes. This will be something that will be hard to forget as time goes on.