5 year relationship and I'm only 18yrs old advice

I've been dating the same guy since the end of 7th grade. I am now about to be a freshman and college and we are going to the same school. We are pretty great together and he's my best friend. I sometimes feel like I am settling to be with him though because I'm not as attracted to him as I once was. He's always been super cheesy and corny, and definitely clingy (which has been bothering me more recently). Every time I think we should break up I feel as though I can't do it, like we've been together so long I feel trapped. He's already planning out our entire future together (and I don't even want to begin to think about marriage, I'm too young!) and I don't want to tell him to stop cause I don't want to break his heart (he's very insecure about our relationship because I'm kinda out of his league). He is basically acting like we are married already and it's just so overbearing. I don't know what to do. I just don't want to lose someone so important to me.

Please any advice would help. Thanks.