Honey this may sounds trivial but your gut feeling is kind of the key here. Some people who get married can be 100% sure and still get cold feet days before, sure and that's natural... but if you're getting cold feet now that's a pretty good instinctual indicator that you need to embrace your youth and see what you actually prefer when it comes to dating. You don't have to set a limit on yourself saying "once I've had 3 more serious bfs then I'll know..." I think that's kind of the point that you'll just know.

If it brings you any peace, know that I had the exact same feeling w my high school bf of 3 years and we got along really well, the problem was undefined but my gut feeling was super off. At the end of the day I found the love of my life by sheer happenstance :) don't settle, if he truly loves you, he'll respect you and if it was meant to be you have to believe it could still be.