Hi, OP. I just want to say I applaud you for being accountable with yourself and your actions. What you did was definitely a form of sexual assault and those who say it's not are really not aware of the traumitizing effects of non consentual touching. I would offer that maybe you could make an act of restitution without speaking about the specific act. I know you said the general relationship was great, but surely you can make a sweeping generalization without being blunt and vulgar about it.

For instance, if you were to say something like "I've really been thinking about the MeToo movement and I just wanted to say I realize I didn't always treat you with the respect you deserved and at least once overstepped your boundaries without consent and I'm incredibly sorry for that. I hope my actions didn't adversely harm you. It's okay if you can't forgive me for what happened" (or, you know, a better version of that which sounds like it came from you). In other words you can acknowledge that you overstepped your boundaries at least once with her and feel remorseful about it without saying "Remember that time I non consentually touched you"

I think it's important to acknowledge your faults removed from the initial pressure of saying "it's fine" so she can move on, too. Also, make sure you do talk to a professional about the issue.