You are hitting your head with a sharp nail and have been doing so for awhile, and are bleeding from self-injury.

That you feel so negative about yourself, so guilty, and so remorseful (over nothing, really - do re-read your text to note that the amount of text devoted to the facts is FAR smaller than the amount of text devoted to your feelings) is a symptom of depression. You need better fitting medications. The depression is putting these thoughts into your head.

Another idea is that what you really feel remorseful about is not the sexual part (which is really nothing but kids' play) but that you were not kind to her many years ago. The answer is to be kind to her now - NOT by bringing up the sexual play, but by listening to what she has to say, congratulating her on her wins when she plays your video games, finding out about what she is doing besides school, etc - you get the point. Build the emotional connectivity between you two now that neither of you is sexually curious or confused.

If you have a therapist, I would bring up how you blew the story out of proportion in order to give yourself material for feeling guilty.