It’s so nice to read a supporting thread, I remember posting about similar problems a few years ago and I got flamed 
I’ve struggled for years with anxiety and a lot of it stems around DSC and their mum.
It started when I was pregnant, I suffered with pre-natal and anti natal depression, really struggled to cope with simple things, at times I couldn’t even leave the house, shower myself, take care of my baby (and DS from previous relationship).
It was a really hard time, DH needed to care for me, I couldn’t stand the extra stress of two extra kids around and having to try and put on a brave face. I thought about leaving DH as I couldn’t be a stepmum to his kids but I needed him and I loved him.
Then to make things worse DH’s ex was constantly criticing us because we weren’t doing enough with the kids/I wasn’t being a good enough step mum/sometimes DH was staying at his mum’s with the kids or needing his mum to help us out.
So DH ended up telling his ex what was going on and then she decided she didn’t want her kids around somebody with mental health problems, making him chose between his wife or kids at weekends.
5 years later things are now much better, I do love my DSD she’s just the loveliest, funniest girl and we have lots in common.
I’ve never really bonded with DSS but he is very much like DH and I care about him a lot and really want the best for him.
But it drives me crazy when they have a lovely weekend with us and then go home and complain about something and then BM has a moan at us.
Their mum is a royal pain in the arse, absolutely control freak who likes to make our lives as difficult as possible and criticise us constantly.
It’s that which causes my anxiety, always having to watch what I say and do because I know it will get back to their mum and never knowing what trick she’s got up her sleeve to cause us more stress.