I did something similar. When I didn't know if I would ever see her again, I started writing a blog with her name. She'd find it if she every searched for it. Just in case. Fortunately, the mom ran out of money and energy and came back.

And even afterwards I always focused on what I could do. There was no help from the powers, and the mom won't ever change. For years, I traveled 16h on the bus every 4-5 weeks to stay 10 days around my daughter, with her. I had some right to see her, as long as I kept paying the child support (which of course I did). I took advantage of that to the fullest, and created an incredible bond that through persistence and patience evolved into Lily coming to live with me.

I did fight, yet not against the mother, but for my daughter. And there are many ways to do so, even writing letters, trying to keep in touch.

It's still very complicated, quite the minefield to be honest, but the more Lily grows, the more she knows, the more she starts thinking of her own and the mom is losing power, day by day. One day, we might just be equals before the child.