How do you know someone is the one?
When you can't imagine your life without them, and not necessarily in a lusty way. A lifetime commitment is a commitment of the soul and of the heart. If this person is such a deep part of your soul that they are your absolute other half and your best friend, then you KNOW it's worth fighting for and worth keeping.
And yes, sometimes you just KNOW. You just feel it. I've been with my boyfriend for 7 years. We work at home. We are around each other 24/7. And I gotta tell you, I can't imagine a me without a him. We knew that after a couple years; these things take time.
Generally relationships built on a sexual flame burn out. The ones that are built on true love, trust and an unspeakable connection are the ones that stay. But that's just my experience.
Remember, there is no selfishness in love. Too many people are way too high school and immature to understand that. There is no pride in love.