Love the one you're with! (SPOILERS)
This just made me think so much of that song. Great, classic song; but I never seem to see this kind of scenario played out in cinema. It is so realistic, I think!
I had something a little bit similar happen to me (except that somewhat unfortunately in my case "the one I was with" was not oblivious as in the movie). I used an online matchmaking service which found me two women that I really hit it off with. There was one though that looked like it was the "true love" match (from my perspective), so although the second one seemed promising too I told her that I had already found someone. She was very upset as she really liked me a lot.
Then the one I thought was the match made in heaven decided that she wasn't really feeling it after all. I went back to the "second choice" and asked if it was too late for the two of us to give it a try. She said that it was not...and, fast forward five years and we are married, have two kids, and are very happy together. It happens!