How do i tell him without being an ass?

So I have been going out with this guy since I was in 7th grade, we are both now in 10th grade. I haven't had any other boyfriends than him , he says I'm the one for him. The one he wants to spend the rest of his life with. I love him a lot but Im bi and I want to date a girl. I've had girls that want to date me but I didn't wanna cheat on him so I cut them off. I've never been with another girl but I really want to. I want to be with more guys as well, I want to live life and be in other relationships. I don't want to tie myself down and say he's the one for me, I want to date other people and see if he's really the one for me.

So question is, how do I tell him i want to date other people, I want to find out if he's really the one for me. How? I really truly love him and he will always have a place in my heart but I don't wanna sound like a selfish bitch