How did you know when you found "the one"?

I've been in two long term relationships and I'm currently in one. A couple of days ago my boyfriend asked me "can you see yourself spending the rest of your life with me?" And the question hasn't left my mind because sometimes I feel like I can't be with him forever, however I am happy with him in this moment and the way things are going. I just don't feel like we're going to be together forever.. if that makes sense. I felt the same way in my first relationship that was of 6 years. I haven't had a moment where I feel "this is the man I'm going to be with for the rest of my life"

I'm 22, and a lot of my friends are getting engaged or some have already gotten married. I wonder why I can't find someone I want to be with for the rest of my life. I know 22 is young, but it just makes me think.

How did you know when you found the one?