Based upon what you've written I don't see why you want him back! Do you honestly feel this is the best you can do?
However if you are determined to get back with someone at some point you have to communicate.
Most likely a face to face conversation is best.
If's tough to get someone back (if they dumped you) because you come off looking like you are begging or worse, "willing to become whatever they want you to become." Either way it smacks of desperation. (Your best option in this case is to wait until the next time he says he wants to spend the rest of his life with you.) At that point tell him lets talk about this in person.
Ask him what assurances is he's willing to give you that he's not going to break up with you again without talking things over. This puts him in the position of trying to "sell" you on taking him back.
If you are the person who dumped him all you can do is tell him what you were going through mentally at the time you ended the relationship. You would have to tell him what you've learned from being apart from him and how you would never behave that way again. Your job is to "sell" him on trusting you to stick by him through thick and thin....etc