I'm literally a loser.

Ughhhhhhhh!!!! I am SO frustrated myself.  This is going to be LONG.  My boyfriend and I have been dating for 2 1/2 years.  He's my longest relationship and I love him so much.  We have a 10 1/2 year age difference. He's had 2 previous "serious" girlfriends and I'm always letting that get into my head and it's putting a cloud over our relationship.  I know it's my issue not his.  I'm always the one bringing it up and unable to stop thinking about them.  He says "it's not fair that you keep bringing them up when I decided to let them go and I chose to let them go for a reason.  I didn't want to be with them and I don't want anything to do with them".  I know he's right and it's not fair of me to do this to him but why is it such an issue for me?  I'm always comparing myself to them and I'm always comparing our relationship to his past ones.  He was with his first ex for 7 years and his second for 3.  I found pictures. Nude photos that he thought he had deleted. Photos of trips and of them kissing. Letters he wrote to one.  Texts messages to another that were saved on his Mac that he thought he deleted I was snooping and that's how I found all of this.  I go on Facebook and look at his second because I think she's prettier then me and I have all these thoughts about how he had so much fun with her based on the trips they took.  So when we went on our first road trip I ruined it because I was thinking about the romantic beach trip he took to Cuba.  Then I get even more down on myself and think wow he probably did like her better because she probably never obsessed or ruined things like how I do.  I honestly don't know WHAT to do with myself.  I love him so so much but why can't I stop comparing myself?  It is so upsetting because I ruin everything fun that we are gonna do.  We live together and I'm the only girl he's lived with and we have a pet together and I'm really close to his parents.  So why is it that I'm not confident enough?