Reading this reminds me of when that sort of thing happened to me a few years ago, and to this day I feel awful that to some degree it still rules over me.
She was someone I trusted, we did like each other and it went on for a few weeks between us, but then one day she started blackmailing me with her own safety (she was deeply into magic and curses and things, she knows I don't believe a thing in them) if I didn't do "said sexual thing" in front of her. The first time, I was a bit more naive and thought she'd really be in danger, so I did. As you say, it feels violating and incredibly degrading.
But, I'm slowly gaining control over it, time has made the majority of it fade. It largely affected my trust and made me wary of getting as close as i wanted to my girlfriend, and to be fair I cried as I told her what had happened all that time ago, because I felt like everything had been taken away from me in that instance.
My point is, it takes time, and even though it's hard, getting it out of your head by writing it down is sometimes the better way to start gaining control over it.