I Feel so terrible... What do you think about this?

Well u can't stress on the past, but maybe talk to him. Let him down easier than just a flat out "no" cuz those 2 letters can cut pretty deep. But, if you don't see yourself ever liking him like that then you hav to get him to stop cuz if he is obsessed with you, this isn't gonna stop. You could try letting him no that he is overwhelming you and its making him uncomfortable, if you still wanna be friends with him then let him no that but it would be better if you didn't talk about relationships with him (like if you find a guy you like) unless he has moved on and is dating someone else. And don't act too close to him or else he'll misunderstand and think u r(or could be) interested in him. Try and make some space between the two of you wenever possible and don't be too nice. Good Luck(^-^).