Stay or leave?

You're still young. People grow apart all the time. It's normal. If you're not happy in your relationship, then end it. Go do things that make you happy. And I promise you that when you're happy, that's when you'll meet someone else. Maybe that next guy won't be "the one" either, maybe they will. It doesn't really matter, honestly. What matters is that you'll be growing, you'll be getting better, and each experience will bring you closer to where you're meant to be, because this clearly isn't it.

As for not knowing what to do if you left your boyfriend, well... To take from Lewis Carroll, when Alice was lost in the forest, she asked the Cheshire Cat "Which way do I go?" The Cheshire Cat responded, "That depends where you want to end up." Alice exclaimed, "I don't much care where!" So the Cheshire Cat said, "Then it doesn't matter which way you go." The important thing is that you aren't exactly where you are right now. So don't worry about the destination. Just pick a direction and go forward.

You should speak to him about your concerns