He, you screwed up after the first time. Your the one he just comes back to because he knows you'll accept him.
Word of warning, being with him you are officially on the highway to hell. Trust me I know. Read He is just not into you. Sorry a quick google will give you the author's name. Very short book but will change the way you view relationships. This guy is not into you. You are sadly his "doormat", he can do what he likes, and you take him back.Words are cheap, actions show more about the person, remember that please.
Why should he change? Honestly find someone else, stay with him and this will be your life forever more. God forbid you bring children into this situation. Get out now.
It means he knows that as you are still here he will continue to treat you the way you are letting him.
It means he doesn't want to tell you anything. It's his secret/good time and doesn't want to anger or upset you,thereby making his time with you uncomfortable for him. He keeps coming back to you because you keep accepting him. You are his repeating transition. He uses you between other women so he never has to be alone. He doesn't know what the word love means and will never actually commit to you, regardless of what he says. You deserve better and he needs professional help. Let him go!
Seems like he may be skeptical of marriage right now, but willing to go through whatever you two have to go through in order to make it permanent. You two have gotten back together 3 x's then there is definitely some strong emotions there! Good luck!
You deserve someone who truly respects you, everyone does.