How do i make my relationship work?

i am 8 months pregnant and my boyfriend and i have been on and off for 2 years now. he has been in and out of jail and still continuing with his irresponsible acts! we broke up a couple months ago cuz i caught him cheating on me, i got back with my ex and that wasnt working out, so i decided i'd try to work things out with my boyfriend. mind you, he had been in jail and wrote me letters promising things would be different. well, i just recently moved to a different town and all we do is argue. i think a lot of it derives from my lack of being able to let things go. its like im never satisfied with how things are. i worry about what he's doing and question him a lot. he's on the edge and almost ready to give up, but i want us to work, not only for me, but for our daughter as well! what can i do to try to be less, overbearing?, and learn to let things go without it stewing over and over in my head?