When...

"I'm standing in the rain, as my hair falls in my face.
It's hard to see, like a hazey window pane....
I can't see in front of me
If in my future it's you I see..."

I've said that if you really like someone, then they are worth the wait...
A question came across my mind recently that brought me back to this certain subject. Patience is a virtue right? But after how many disappointments, hopes crushed, and heart breaking events does the virtue of patience turns to hopelessness.
This, I do not have a perfect answer to, or an answer at all. When you are young you never know if the person you are falling for is the right person. Because, you will fall in love, break your heart, and cry numerous times in your life. It is just life, cause you never know if your falling for the right person till you give them a chance.
This though, does not leave my question answered.
If you fall for someone who at the moment does not have the mutual feeling, when is it worth holding on, and when is it time to moving on?
How can you ever be sure?