Love can cause pain, but only as an activation of negative feelings, real pain (talking about body wounds) instead has always an event trigger (slice, blunt damage etc.) which stimulates your synapses with an electrical flow. So we have to seperate the pain you feel or might think you feel in your "heart" and "soul" or the body driven one.
Let me answer for the "if love is real and letting go etc" from my very personal point of view. I had a girlfriend quite long ago and I was really madly in love with her. We couldn´t become a couple because of some reasons and there was no right way to get this done. I was surely in the mood to die for her, for a last word of her, her eyes gazing upon me just to guide my way to Valhalla (or so). And I had to let her go because it was a deadend. But letting go is not a rational decision in this position, it is simply the realization that you have to give it up. So you don´t let someone go because you wan´t to, it´s because you know it can´t be done and you are giving it up. This causes horrible pain (this is the worst pain I EVER felt!) and therefore love and pain are for sure combined. Everybody who felt lovesickness will nod his head now. And I realized that both of those feelings just raised my understanding of how I feel love for someone or not. Both are necessary, sometimes you have to suffer to know, why you do so. But love and even pain still give you power to succed in things you never thought of. How about writing love poems? Mostly they aren´t written while you are in love, they are created because of your suffercation.
Love hurts *sings along*