"All of this hard work and dedication didn’t pay off. "
You may be wrong here. It probably did pay off but it wasn't the payoff you expected. Maybe it did not give you the outcome you wanted but your kindness probably helped her with her recovery.
"I know I need to let go"
Yes you do. Trust me. I have been there. At least in my case she never came back and all I did was extend my own heartache.
If you love something set it free. If it comes back it’s yours. If not, it was never meant to be.