This happened to me once. It was a while ago, before I began therapy and understood that I dissociate and have a system of sorts. I'm sorry it's bringing up so much pain for you....

I have a thought and you might have already tried it. Is there any way you can safely invite this alter into a context that isn't sexual? Have a chat? You might want a therapist's help if you haven't done it before. In my experience, the parts that have been uncontrollably disruptive in the past become gradually less so once I actively invite them and try to hear them. Sometimes it takes a long time for them to come out this way, though. And the things they say can be very hard to hear.