I know it's very very hard to let go of someone you love, but in the long run if he keeps breaking your heart, he'll wear you down until there is nothing left of you. Be assertive, do the right thing for yourself and say that's enough. Move on. Take some time alone and remember what it feels like just to be happy.
When you are ready, you will meet someone who will change your life so dramatically you will never even think about leaving him. You don't deserve to be unhappy. It'll be better for both of you.