How can we know for sure?

When you let love go......because the two of you just can't seem to get it right. When you fight and fight and they make you feel awful and you think "Why do I stay in this relationship!!"...BUT...when its time, how do you know for sure your doing the right thing. Other people can convince me all day long. "You don't need him" "You deserve better" "You guys are both great people but your just not made for each other" But they aren't there in your most intimate amazing times with said person AND they aren't there when your at your lowest low, in your darkest hour so how do they know whats right? One of my friends said to me tonite "Unless both people are willing to give 100% to that relationship it won't work and it wasn't meant to be" If this is true then nothing I have ever done, no one I have ever loved was anything real. He played a song for me called Brown Eyed Blues(not FOR me but just so I could here it) The lyrics go:
"She’s got those eyes, those eyes
That’ll see right through you
When she leaves that night
I wanna leave, with her too
And she’s on my mind
And I go all the time
When we touch I go weak
And I can hardly speak
And I hope that she thinks about me
Cos I’m always thinking of her

I just wanna hold her hand, be her man
I wanna know if she’d take a chance
Cos I still have not revealed it
Cos I still get the feelin
That lovin her is a game I’ll always lose
I got the brown eyed blues"

And it hits me that it is possible for a man to feel this crazy about a woman but I have never been that women. No one has ever felt these things for me. Should I be hoping for this or is the real world what I experienced for the last 8 years, is that the strongest love I will find? Did I make a mistake. Should I try to get him back>? Or does the love in song lyrics romance novels and on cool summer nites even exist.....Of course it does but does it exist for me?