She is quite picky. I'm not sure if thats the only way but when I tried for her only one way works well: teach her self-respect (instill that her personality is important, not just her body) and be chaste with her (do not fuck her at all). Well, and teach taste in clothes and wean off swearing of course. In this case, she gives up on the idea of being with her sugar daddy and is ready to consider the option of staying with MC, but only if he refuses all the other girlfriends. That is, you will be able to continue working with clients and pursuing side girls, but no other girlsfriends with unique scenes will be available except her.
If you do the same as I described, but fuck (without BDMS) her at the same time, then she will end up with her sugar daddy and you will never be able to call her again. If you put her through BDSM training, she still won't be able to become your girlfriend, but sugar daddy will allow you to visit her sometimes and fuck her. I'm not sure if these are the only ways, but this is what once worked for me. In my current playthrough I gave up on her, it's much more fun to have a whole harem of girlfriends at home instead of just her.