It has something to do with your idea of family, you've mentioned that you can't cut your mother out of your life because she is family which would indicate that on some level you wish to create at least a decent relationship with her or at least thats what you're trying to get to whether you can/will or not I'm not so sure because you are extremely scared to be hurt again by someone you care about and you know because of this internal need to get some sort of resolution with your mother she can use that to hurt you deeply (it seems like something of a moth to flame relationship).
Although your parents have split up for the better you still love them both and you have come to terms that them divorcing was good for all of you because of the reduction of conflict. Unfortunately one thing that you hadn't factored in is that they might remarry which although you were perfectly aware that they could or even might you probably thought that it would be at a long time off during which time you could establish a good/better relationship with both of them and feel like you're more part of the family, more included than you currently feel you are. It's possible you may even feel like time is running out in that regard.
I think it's possible that because of the nasty divorce process that you went through you essentially developed quite strong abandonment issues which forced you to over compensate and become very rational to prevent emotional hurt, it also created this desire to belong to a family unit which is why you've developed a sort of surrogate family of friends and why you are extremely loyal to them. Unfortunately I think the severely abusive ex boyfriend not only strengthened these issues but expanded them to encompass any deep relationship that you could make which brought along trust issues to accompany the abandonment ones which could give partial explanation to your sexual preferences, S & M practices involve a lot of trust between the two people and can actually help with trust and abandonment issues because of the dominant and submissive nature and the high level of trust that is required between them.
I believe that the reasons you're struggling to come to terms with your mother marrying again is because you think because you don't get on with your possible step father, if this happens, you will be cut out of the family all together, that you'll never be able to establish the kind of relationship with your mother that you are trying to and you will never feel like you are loved and needed as part of the family.
I hope this post is okay, I would like to point out that I don't concretely stereotype you as a person and I'm aware that if I list off things in this kind of manner I can appear quite harsh when it isn't the case. I've went through what could probably be called a watered down version of a divorce in comparison to what you went through but it carried over a good number of years, since then both of my parents have remarried and had children and although I'm still on good terms with my mother, my father and I don't really communicate for several reasons, however over time I've tried to re-establish some sort of relationship with my dad which has been.....semi-successful by an occasional meal at a restaurant instead of a place where he has the upper hand (communicating with my dad is like a game of chess) and after playing through the general formalities of "how are you?" and so forth to keep the focus on the other patrons.
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Sonyab; It does sound selfish but that's due to the nature of the issue and DevilDoll's past. I'll give you an example in regards to my situation: after a period of time both my parents started dating again, some of them I liked and others not so much, I found their presence imposing, like they were trying to take away my connection to each of my parents and that I would end up being something similar to a lamp but I could also see that some of them made my parents happy so I didn't know how to deal with it. When you are young and go through something like this it can be something like a shock or you feel like you've been cast away or replaced, at a young age you do have a very self orientated mentality and things like a nasty divorce will affect you into later life whether you like it or not, the annoying/interesting thing is you seem to on some level keep that childlike mentality. Another example would be (stealing from a film to which the name remains elusive) imagine you were born in cavemen times but never died, stayed in perfect health throughout time, you could remember snippets of the things happening around you, you could remember some of the valleys and areas around which you lived but come the dawn of maps you wouldn't know which area became Africa and which area became Europe but you still remember the appearance of certain valleys distinctly, you remember the emotions that flooded through you when you gazed upon the sun rising over certain mountain ridges, you'd be left with a desire to experience that again perhaps but wouldn't know how to get there and as a result you would probably be left with the desire to travel a lot but not entirely know why. If you flip that to be regarding a negative experience and add to it that it's quite likely that DevilDoll watched other kids that were happy with their family and wanting that for herself it makes perfect sense so I wouldn't say it was selfish more that it's natural and completely understandable. I would be willing to bet that DevilDoll was like a mini adult as a kid because of the stuff she went through, knew what she wanted and didn't want quickly, quite observant and outspoken too.