Retroactive Jealousy - Threatened, Insecure, and obsessively jealous of my boyfriend's past relationships.

I am suffering from acute retroactive jealousy. I get obsessively preoccupied with my boyfriend's past relationships and constantly compare myself to his ex girlfriends and feel horribly inferior. He is a very understanding and compassionate person who constantly reassures and supports me. I feel like a horrible person for being so obsessed with something that happened years back and I constantly feel so immature and superficial for being this way. I know he doesn't deserve this. I know this is so unfair to him. I feel this way in terms of his sexual history as well. I feel as if he was sexually more active and intimate in his previous relationship and he is not so much with me. I feel so pathetic and depressed all the time for continuously comparing everything and being perpetually jealous. I have never been a jealous person in any of my previous relationships. This is my first time encountering something like this and I feel so helpless. I have been trying to overcome this for almost 1.5 years but all in vain.