What does it REALLY mean to be IN LOVE?

Im still trying to figure this out. I know what unconditional love of a child is, because I have one, and I know what that feels like, and I would do ANYTHING for my child... but as far as relationships, I just dont get it. What does it FEEL like? I'm in my late 30's and still haven't figured this out. I have felt like "maybe" I have been IN LOVE a few times, but then I feel like after I break up with them I can move on so easily, that it couldn't have possibly been love. What exactly is it? Is it the feeling that you would do anything for a person and that you feel as though you couldn't live without them?