How painful is it when u just can't stop being in love with/loving someone you know you can't possibly be with?

I just can't stop myself loving this girl, even though I know I can't be with her. She's been with someone for a long time, & I've been pretty obvious in my feelings with her: she knows everything. We're (platonic) soulmates but not lovers. She confides in me things she can't confide even her bf in. I just can't get her out of my mind, I write poetry thinking of her. She recently moved to another country & wrote a note for me before she left. I read it everyday. I don't even need to be with her to love her. I don't regret not being with her; there is sadness & pain inside me, but oh so beautiful.