I haven't had to watch her love someone else, but I've had to sit back while she dates multitudes of people whilst I'm unable to move on because of my feelings for her! But you know what? Time may not heal it completely, but it definitely starts to numb the pain after a time and you just grow to accept it for what it is. Just pick up the guitar and play man. That's the only companion you need until the right one comes along!
I hope you're fine though...