Ex sent me this letter, how do i respond?

It sounds like he has realized what an amazing person you are and regrets his decision to end it with you. There were clearly problems in the relationship for him to walk away, and that is something you can't take lightly. It sounds like he sees you as moving on and being happy and satisfied and like he said "it kills me that you may have found happiness without me". You have fought long and hard to move on past this devastating event, and you deserve the fruits of your labor. Keep seeing the new person, and if that doesn't work out keep seeing new people. Find out who you are and what you want from a partner, because now is the time. You may find after dating some people that you do want to be with the this guy, but you may learn that he is the exact opposite of what you need. Ask him to respect your right to live your life, and if he is worth his weight, he will respect that request. If not, that tells you that your feelings are not a priority for him. If you are meant to be together, you will. Good Luck!