Please help me, drowning from childhood guilt :(

You said you did this only once, and obviously you are horrified, and filled with remorse. This is a good thing for you. True child molesters do not feel remorse for their actions, or if they do, their remorse is short lived, and they do it again, and again. Sounds to me like you have this thing handled pretty well.
Hang on to the remorse even if you forgive yourself. Remorse is necessary, believe it or not.
Remorse is a reminder and can come in handy as a ' warning tool. '