Sorry you’re feeling down. Yes, you’ve got to keep going. It’s wonderful that you’ve found the strength to be busy and active.
With your kids, you can try to counteract your ex’s negativity with positive affirmation. That’s the thing I’m most grateful to my mum for. She’s long gone now, but throughout my childhood she repeated the mantra that I could achieve anything with hard work and preparation.
My DH is from a broken home (FIL left for the OW). MIL lacks confidence and esteem from her own childhood and she passed on all of this to DH. It’s really sad. I find it exhausting trying to counteract the damage his parents did to his sense of worth. E.g. we used to have big parties before Covid and every time he was convinced that no one would come. I have no idea why he’d think that, but he did. He’s full of ideas about limitations that he couldn’t do this, that or the next thing.
Our young DC wants to be an astronaut. DH will say that won’t happen. I say “well, that’s a wonderful ambition and these are all of the things you could do to prepare for that career”.
Anyway, I suppose the thing that my wonderful mum did when I was a teenager was apply a sort of coaching method to her advice (eg. What do you think you could have done differently? Perhaps next time time you could try x or y or z etc).
I loved getting cards from her with things like “If you try to reach the moon and fail, you’ll still land amongst the stars”.
Wishing you all of the good luck and strength to get through, OP. Greatly admire your courage