What more can you do if he doesn’t love you back?
You’ve done all that you could. Be his best friend, always be there for him, always listen to him whenever he needed someone, worry for him, but it’s all not enough.
You can’t end this because then you’d be giving up, and giving up means you give up on the idea that he’ll love you back. All of your time and energy would have been for nothing. You can’t end this because you’ve invested too much, because your heart won’t let you.
And you can’t give up because, regardless of everything that’s happened, you just fall more in love with him.