I dont want to be with my boyfriend anymore what do i do?

We have been together for almost 3 years and im 7 months pregnant with his daughter...i recently moved down to nc from new york with him a month ago hes so happy and excited about the baby coming but im not happy with him and i dont have the same feelings for him anymore i feel so guilty feeling like this especially now that we have a baby on the way but im miserable with him and he just annoys me most of the time acting immature hes not perfect he has lied and cheated on me in the past but that alone doesnt make me have the guts to tell him how im feeling. I dont know what to do I dont wanna break his heart but then i dont want to stay and be miserable how can i do this in the nicest way?