Here's something useful! I was having a hell of a time getting the achievement for breaking a girl, so I made one who was easy to break. I took the Mavis from earlier in the thread, changed her outfit because I didn't like it (top was wrong or I didn't have the right clothes, and skirt was too light), and then set her personality to tomboy/fragile/sex hater.
Info on the positions and their effect here:
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I think you get credit for each position once per encounter. So you can't just do forced missionary 8 times in a row, you gotta mix it up. Still, in like 6-10 encounters she was broken. She really hates the MMF positions, that helps.
Now I'll likely reset her because I feel bad.
There's no specific reason it was Mavis that I used, I just picked something from my folders that I hadn't used yet.