I have a similar past and have found letting go to be a huge relief...once I got there. You can't rush into completely giving yourself over, knowing that they will have the opportunity to hurt you and there isn't a thing to do about it. The thing that finally did it for me was actually a bit of discipline. It was the first time I had earned a spanking and the release of tension I got when I was bent over his knee triggered my ability to let loose in later sessions. That act let me know that he was really just there to help me grow as a person. It's different for everyone, I'm sure but since we share a similar background, maybe your trigger will also be similar.

I wish you the best of luck!!