It sounds like you’re not really ready to have sex. If you were, you’d stay. Instead, you’re agreeing to have sex, but then you leave in the middle because you’re not in touch with your true feelings. Maybe you’re trying to have sex because you don’t want to disappoint your boyfriend, or maybe you want things to be different than they are so you try to start it up, but as sex progresses, you stop it and you walk out because something is bothering you. You may be fearful of pain or of making a moral or social mistake or maybe you have some other feelings that are bothering you about having sex — but the bottom line is that you’re leaving because some part of you wants to.
My advice is to take a break from sex — you’re trying to force the sex and the relationship, and it’s not working for you, or your boyfriend. So just hit the pause button. Figure out what’s bothering you — because it’s something, and you’ll have to do the work to understand what it is. Not everyone is ready to have sex at the same time, so respect what your body is letting you know and wait until you’re ready.
I hope that helps. 😉