I can't say I've broken up with a pothead.. but I've been a pothead. I quit almost five years ago. It's irrelevant, but I think you made the right decision to leave him. I would NEVER date someone who smoked pot. It's illegal, and I don't care what people say, it's a gateway drug. From my experience, nearly every pothead I've met is just a loser. Harsh, but true. I was in fact that loser, until I decided I wanted better for myself.
I would stop trying to worry about the past. The I wishes, should of, could of, and worrying about what if's. It's done. Not to sound harsh, but it will be the easiest way to move on. I know, it's hard...
I'm sorry that you're heartbroken. Do you have any hobbies yourself. What do you like to do? Do you have positive, healthy friends that can offer you happiness and distraction? I would do everything in your power to just focus on you. You left him for a reason, try and focus on that.
I wish you the best honey. I know it's not easy. I don't know your whole story, so it's hard to offer much advice on it. I just truly believe potheads are a waste of time and energy. A lot of my friends still do it, and that's fine. I just fine myself wanting to see/talk to them less and less. You're better than that. Call me cynical or critical.. drugs are just stupid in my opinion. Pot included.
best of luck!
