You're angry at your brain, but gosh, it's doing a really good job trying to protect you! It's making sure you aren't asleep with a man, which would leave you very vulnerable to being hurt.

Of course, the irritating thing is that intellectually, you realize you're safe in bed with him. But on a gut level, your brain does not yet understand. I have found it helpful to stop criticizing my reactions (which are endlessly embarrassing and disruptive!) and instead to sort of salute them. They are my very best efforts, in the moment, to be safe. Reteaching safety to your brain takes some time, so be patient with yourself. You will learn to go through the fear rather than around it.