I cannot fathom any response someone could send to the mother of their children in this situation that could be anything other than at least minimally compassionate.
Once again, proven wrong. Ugh. My god there are some arseholes out there. You aren't a burden OP and there's loads of us happy to be your cheerleaders on here.
You and your kids sound lovely and they seem to be dealing with this in a very grown up way and I worry this may help their dad kid himself they are ok at times they may really need his support.
I hope their dad realises that it doesn't matter how grown up children seem they are still his kids and that he checks in with them about this situation so they can ask for help if they need it.
After years growing up in the warfare that was my parents relationship (different situation I know but could help), one of my mum's friends gave me a piece of paper with her phone number on.
Back in my NOKIA 3310 days... and she said that she knew I tried to be grown up but if I ever needed to chat, ask for help, speak to my dad about something that I was scared to ask him, tell my mum for me that I'm sorry for being moody I'm just worried about her, ask questions, ANYTHING then she was on the other end. To text, call, come meet me in a safe place.
It's one of the kindest things anyone has ever done for me. Maybe another adult in your kids lives could be their safe place in case they are sad or if they are worried about upsetting you too as they sound lovely.
Wishing all of you the best 
