Ep, I'm sorry you are struggling. You are right to focus on your child, let H blow in the wind if he must, but keep things are stable for your S as possible. You are doing a good job there, keep it up.
I really struggled with GAL in the beginning. Slowly, I found new friends (most of whom are single), and found some things I like to do (paddleboard, snorkel) and now I have to say I'm pretty good at it. What size city do you live in, do they have a Meetup group? Meetup has been great for me.
The other day, after I asked to proceed with the D, I asked STBX if he was happy, if this had turned out like he wanted, if he was happy with his R with his children. When he said, yes, he was happy with his R with the children, I'm pretty sure I had a horrified look on my face. He's happy with seeing D12 35% of the time, and taking D17 to dinner every couple of weeks. I will never understand how that's OK. Ever.