It sounds like your husband is either clinically depressed or having a mid-life crisis or both. What jumps out at me here is a sense of mystery. I don't have a clue why he left and I'm not that he does either.
So the first thing to do is to get him to identify what's going on inside himself and to put his thoughts and feelings into words. If you want to keep the marriage, then I would tell him that while he says it's over, he acts like he still wants to have a relationship with you (he calls twice a day, and wants to celebrate your anniversary).
I would tell him that he shouldn't make any decisions until he sees clearer into himself. Ask him to come with you to a marriage counselor. Get him to talk about what's going on in his head and heart. What made him unhappy in the marriage and what made him feel dissatisfied with you.
If there are things that you were doing that were rubbing him wrong or things that he wanted that you weren't doing, thank him for telling you, then ask him to give you a chance to be responsive to him. Encourage him to keep on talking, not walking. If his distress is internal, then encourage him to also get individual help in order to sort out what's bothering him.
Good luck. I hope that you can work it out.