Hiya friendly guy.
I'm sorry I know it hurts.
A couple of things to ponder (like the cat in the hat)
Thing one:- If you get to have the credit for "making her better" are you ready to take the blame when she flounders or fails ? If she is going to sort it out she has to be the person who takes responsibility for that.
Thing two:- (Someone at my AlAnon group taught me this and it makes sense to me) Although you said you fell deeply in love your feelings don't come from Sarah they come from inside you. It may have been her who opened the box but the contents are all yours. All it means is friendly guy is capable of feeling love. If you think this through (to me) it means the opportunities are there to experience these feelings again, perhaps with someone new or perhaps with Sarah at some point in the future when she has sorted herself out.
I wish you luck