Should I breakup with my fiance?

Well I am 26 and i have been engaged for a little over a year i have been with my fiance for almost 4 years. We met during a lonely time in my life and we found each other online. When we first starting talking he didn't really seem like my type for a lot of reasons, plus he is a few years younger than i am but i could tell he had a good heart and was a kind person. We started talking all the time on the phone, like when we weren't working or sleeping we were on the phone. After a few months it became a little more, he said he wanted to be with me and said he loved me. I on the other hand thought he was nice and it was just a long distance thing ( he lived in another state) so i never really thought we would meet. Well as the months ped i started liking him more and more and i believe built up a fantasy of what things would be like if we got together and i liked that feeling of an opportunity to finally have someone in my life. I had never had a real relationship before. So anyways we decided he would come visit for a week and see how it went. I have to add that i have always had a struggle with self esteem issues and never felt like anyone would want me, so i was so excited someone wanted to meet me. So he came to meet me and we spent the week together and he wasn't exactly what i had built up in my mind. Definitely more immature than i am, he is kind of a slow learner but i told myself those things are not as important has how kind he was. I felt totally safe with him, well once the week was almost over we didn't want to leave each other. (sorry this is so long) He ended up moving in with me a month later. We now have a 6 month old son after 3 years being together and we are happy for the most part. i know he is happier than i am. I love my son more than anything and would never change how he came to be in my life. However during my entire relationship with my fiance i have had doubts if we should be together. He is completely sure about us and always has been but i always question us. I don't feel any spark with him i don't think i ever really did. I'm not ually attracted to him even though he is really cute. i never want to have . Im constantly fighting with myself because i feel like he is the only one that would ever put up with my depression or moodiness and i love that he is so sweet and a little immature but i hate that he is a little slow i can't have a deep conversation with him he doesnt challenge me, sometimes i feel like the adult in the relationship. I dont know what to do i have been thinking about this guy i used to work with and had a huge crush on and we were kinda close to getting together until i met my fiance and it just never happened. i think about him a lot he has the things that my fiance is missing. we live like 2 streets apart and i secretly wish we see each other out at the grocery store or something. So what should i do? suck it all up cuz we have a son now and stop being selfish but continue to be unhappy or break it off and see if i can find happiness elsewhere even if its not with the crush?