Absolutely, its okay to get out of a relationship. It is not easy, as you know, I guess.
Killing yourself won't change her mind, it sure doesn't seem right for you buddy, even though you do feel like it.
Most people find they actually "come out the other side" once they have moved on. Its hard to believe now.
Maybe there are physical ailments at play here too - this website caters to finding those answers.
My latest initiative was to cast off all the relationships that were dragging me down. That did include family members, which is radical. Not to play your situation down, but this girl isn't even family, let it go seems like good advice.
Other women want their chance at you.
Let us know how you are.