Should I tell him the truth?

I went out with my mates a couple of days ago, we just hung around at the park. One of my friends (Sean) who i have known for 6 years, used to like me when we were 12, (im 17 now) but i didn't like him, infact i hated him. Well ever since he has always looked out for me, like he stands up for me when people bring me down, he watches out for me (i am an extremely clumsy person), just yesterday one of the guys kicked a mive rugby ball in the park and it went flying towards me, i was just like "****" then Sean came out of nowhere and caught it beore it smacked me on the head lol. We text nearly every night, and i could talk to him for hours and not get bored.The problem is i like him now! I dont know if he feels the same way or not, his best friend told my friend that sean really cares for me. But that was before christmas, another BIG problem is before I realised i like sean i liked his mate, Joe. And we became good friends, but nothing happened. Joe was kind of angry the other day, because Sean and I were so close. He seemed jealous and then he turned his back on me when i tried to speak to him. Everyone says Joe likes me, but i won't believe it until he tells me himself. I like Sean more than Joe, but we can never be together because him and Joe are friends. So, that sucks. Plus Joe is possessive, he went out to break my last relationship up, and it worked. He also hates it when i talk to other guys, but he flirts constantly with other girls,then he would come back to me when they left and be like "i just had a nice chat with so and so" and he sometimes leaves me alone when a girl he likes appears and he would go off and talk to her, then afterwards he'd come back to me and flirt etc. also, on many occions him and his "girl mate" would talk about me, Joe tries to make me jealous alot also. I want to tell Sean i like him, but i dont kow how he will react, its really scary, what do you think i should do? also, how do you think joe would react if he found out i liked sean, i think he would attempt to break us up like he did before.