How did you know your SO was the one?

My boyfriend and I have been together for almost 3 and a half years. He is my absolute best friend in the world, and we have had our ups and downs but always come out strong. He is funny, intelligent, and I know that if I really need him by my side he will find a way to be there. Recently though, I've been losing faith in the idea of finding "The One" because something seems off between us. We are long distance, which certainly doesn't help. I love him dearly and I genuinely think he's am amazing person, but I just can't seem to shake the feeling that we're not meant to be together. For those who have found your "one" person, when did you know for sure?