You're asking such a good question. It is truly a heart-wrenching decision you are facing... made even harder because of a little one involved. I really feel for you!
Through my work with Focus on the Family, I came across this article that gives some stats/info on how divorce affects kids... might help as you think through what to do? You can find it
here if you'd like to check it out.
Obviously, I don't know your whole situation, or the factors that have led you to decide to break up. But, it might not be a bad idea to seek out a professional counselor who could at least give you some outside perspective and help as you make decisions and go forward from here.
I hope you get some good counsel and answers... best wishes to you!