Ho do I handle this? I'm torn.?

Love is a complex emotion, something people go their entire lives without truly understanding, yet somehow i find myself about to jump to it at 18 years old. There is this girl I know, and her current relationship confuses me, to no end. She has moments where she's happy, but almost every day she has a devastating, heartbreaking moment, usually brought on by her boyfriend, to which she comes to me for comfort often enough. We've known each other for a few years now, but only recently have I realized the extent of what I felt. I approached her about it, but was told very nicely to back down; not to make any advancements. Still, she sends me signals that seem to be pulling me back in. In all honesty, I would do whatever it takes to make her happy, its not about having her myself. If I could make sure she was happy with her current beau, and never be able to see her again, I could live with it, as long as I could see her smile once before I did. I just don't know, how can I handle this? It kills me watching her in so much pain with him, and even more knowing if I interfere, it would most likely hurt her worse. I'm stuck at a point with nowhere to go, and would step in front of a speeding train if it meant getting what's best for her. Anyone have even a slight suggestion? She literally feels like my everything. I am not saying this lightly; usually I am sickened by the lax use of the word love, but I guess the tide turns every once and a while.