How to let go

I've been with the same man for four years just got engaged last year. We have a newborn baby and I'm 22 years old. I know I deserve to be treated better and not be with someone who is constantly putting me down or who makes it seem like I can't be insecure and thinks my insecurity is annoying even though he cheated on me just a couple months ago. I'm emotionally exhausted. I just want to end things but I picture him moving on fast and it breaks my heart. I think a big part is I'm also so scared to be alone. This is my longest relationship the only guy I've ever lived with and my sons father. I really do love him but I know I deserve to be treated better. Any tips on how to get used to being alone. I always say I'm going to leave then I let him sweet talk me back. I need to move on and be strong for me and my baby. I just don't know how