Hi there.

Currently it seems to me you just have a fetish on her underwear, and that is not that bad as it sounds. I've heard quite a lot of men have a turn-on for sexy clothes.

I would try start dating a girl and then later imagine her in the same underwear and in the same "prisoner fantasy". Maybe your subconscious is overly active at this age, and it gets overly sensitive. If you start dating someone attractive it might change things for you.