Your counsellor from back in 2019 sounds rubbish, if their suggestion to this situation was part time work!
It sounds horrible, I really feel for you. But holding down a full time job is not the issue here.
Recognising that this is utterly toxic and that you have to walk away from this shitstorm of a “job” and how to do that, is what your counselling should have focused on!
There are things you could do... be assertive and refuse to do anything related to his business outside of set hours for example. And being prepared to take the short term shit over that - which wouldn’t be worse over the long term shit of FIL’s demands! But... it’s such an utterly toxic situation, that I’d say there’s no point.
Go get another job... but why not talk to a decent counsellor about getting your self belief back and making that a full time one?
Are you going to let some bullying arsehole make you think you can’t cope with full time hours? No!
You want part time so that if that’s also toxic, at least you’re only facing it for 4 hours a day.
That’s the wrong way to look at it... if a job is shit for you 4 hours a day, you take the same action as when it’s 8 hours - you sort it (if you can) or you go elsewhere.
Please, don’t look for 4 hours a day with the mindset that you’ll grin and bear the shit cos at least it’s part time.
Go do a job (be it 4, 6, 8 hours...) where you feel valued and enjoy it. Or at least - it’s neutral!
That’s why I’m
at your counsellor previously.
You can prioritise a decent counsellor though. The last year, surely you’ve been working at home for much of it? So you book your counselling that’s only available 10:00-18:00 and you turn off the devices that arsehole FIL is hassling you on for an hour. Take control. But... the whole world has learned how to move online... you can get counsellors from another area who work online!
Bottom line though, yes, your self esteem has been battered but there’s actually nothing wrong with you. It’s all external - your FIL is an arsehole.
I say, go for the part time job if that helps you get out, but do it with an eye to turn going full time. You should be compromising with part time as part of getting out of your current role because your FIL is a piece of shit - not because you want a puppy,
In the meantime... boundaries in your current job. What’s the worst that happens? FIL sacks you for (a) working set hours and (b) putting the phone down on him when he unfairly blames you? Well, that gets you out of the shitty job, doesn’t it?!!