Honestly I think the bigger question is do you know why you're in this relationship? Its pretty easy to see his reasons. Based on your description, this person pretty much does and says whatever will service his moods at any given time. And it doesn't seem to matter whether it's a moment of love coupled with visions of future happiness or the opposite of gloom and complete regret. What you need to understand is that both expressions are self-serving in that he either wants the credit for the perceived value in the relationship or feel pity as being the victim for some imagined harm. He sounds very narcissistic.

Ultimately his reasons are his own, which means you have to control what is yours to manage, which is yourself. If he can cover this emotional spectrum as stated without any rational map that offers just cause, then you have placed yourself on a ride that only stops once you decide you've had enough. Caring for someone is often the hook that we tell ourselves to stay in these situations. But ultimately the reality resides on when you've had enough and understand what you feel can not change him. If he wants something bad enough he will work on making those changes himself. You sitting there taking it only empowers him to stay as he is. He will never view that as a sacrifice. Stand up for yourself and maybe then and only then will he see the error in his ways. But you can't do it for that result. You have to do it because you love yourself first.