I thought we were 'just friends' until she left and I realized how much it hurt.

She and I weren't dating, but we were more than 'just friends'. I didn't want anything serious, and I didn't think she did either, but a few years and several bad relationship experiences later and I miss her and realize she's all I really want. I wish I had taken what we had a little more seriously. I need a song that expresses that. ...-ish. lol

Together - NeYo
The first song I came to think of. A slow and wonderful song, one of my favourites. Specially because of the text and the music it's great:

I'm gonna be the love that's gonna last
And be the one that got your back
Ain't nothing never that bad that we would be together
And though we both made our mistakes
And some we never wish we made
But we'll be okay if we just stay together

I wish you the best of luck

Gavin Degraw :: More Than Anyone.
Chris Brown :: I Wanna Be.