If you have a history of SI, you've probably overcome a lot of instincts peripherally related to self-preservation and pain. It seems clear from your description that these are accidents that happen while your mind is on...other things I'm sorry if they're causing you and your partner(s) stress, but I suspect it's not fundamentally different from those of us who give ourselves splitting headaches when our head gets smashed into the headboard You're just a little less careful with your body than average, and that's ok. One might also characterize an orgasm as the ultimate loss of control, and there's something you used to do when you felt out of control...
I suppose I don't have any good answers for you, but it makes sense to me, it's probably not something to be too scared of, your regular forms of therapy should address it