Can't figure out what's wrong with me :(((

I've been in a relationship with this guy for a really long time now. We started off great friends and then got together. He's the kindest, funniest, most caring person in the entire world and I love him with everything in me. I know he loves me so much too. Before our relationship started and we were just friends I always knew he was in love with me and I lead him on multiple times but then I finally realized how I felt about him and told him. I guess because of me originally not having feelings for him while he was madly in love with me it automatically gave me more power in the relationship. It's not that i took him for granted, because I knew how amazing and unique he was, but I didn't express hmmm gratitude enough for him to realize how much I love him. Anyway, one day we got Ina. Fight and I went to a party without him and hooked up with a guy. I told him first thing the next day and he was very mad at me but then I went to his house and I talked to him and wrote him a card and apologized. It made him realize how much I really did care for him. We were lying down on his couch cuddling and he told me "how could I ever break up with you" the next day he broke up with me. He said it wasn't at all because of that and that he just wasn't feeling a connection but i kid you not five hours later I get all these missed calls and texts from him saying how this was a huge mistake and he was so sorry. Of course I forgave him (this was about a month ago) and we got back together but every now and then something happens and I just start having this idea in my head that he's going to break up with me. It gets so horrible that I can't even focus on anything else and it truly affects me. I don't want to go on thinking about the bad things to come, I want to live in the moment but I don't know how. I'm trying to figure out what triggers these feelings of anxiety. He is away on vacation with his family now so I haven't seen him for a week.. am I getting this mixed up with missing him? I just need someone's opinion on how to move on from feeling so insecure. I want to be able to have the power in the relationship again. I don't know what to do and how to stop feeling this way.