how to win him back

Fancy Nancy
I know this is probably corny and cliche but I need to win him back!! He's everything to me. His ex gave him an ultamadium and said either be with me or stop seeing your kid. So obviously as unfair as that is, he chose to be with her and put his daughters happiness before his. Totally get it. But he doesn't want to be with her. I know from the bottom of my heart and soul he doesn't and wants to be with me. So how can I win him back? Just sit back and be there for him and he'll come around? Cause I'm so afraid if I do that, he'll forget about me and he'll just stay with her. HELP ME PLEASE!!!