Hi OP,
I experienced a very similar thing recently. Also lawyer in pp, similar PQE. I came back from mat leave last year and struggled to get back into gear. This coincided with having a new team partner / boss who basically undermined and criticised me so regularly that I lost confidence and began struggling with brain fog and second checking everything I was doing. This was after a previous partner had ground me down by being micro-managey.
I'm a perfectionist anyway and struggle with MH issues, but this new boss was like nothing I'd ever experienced.
I eventually had time off sick, negotiated an exit package (not much but enough to build an emergency fund) and have now moved in-house and sooo much happier. Still working through confidence issues but everyone is supportive and I feel valued and respected and it's amazing how my confidence has bounced back overall.
My only regret (slightly, sometimes) is that I didn't give pp another go in another firm, because then I wouldn't have a lingering feeling that I wasn't good enough for pp. However, as others have said, if there is an individual gunning for you, it's pretty pointless trying to stay there. Well, perhaps not if you have the confidence and staying power to go down the grievance route but I didn't want that, and I doubt the firm would have sided with me anyway.
But all in all I feel in-house is better for me - great holidays, DB pension, more relaxed culture and more interesting work (even if it's a bigger caseload). I'm pretty sure there are loads of commercial in-house roles out there so perhaps that's a route to consider.
I applied for several jobs and got almost all of them, which was also a confidence boost, so hopefully that'd happen to you too - don't worry about interviewing after a long time, it'll come back to you and there are loads of online resources, and best of all just practice with friends / family.
I'd be really happy to talk about my experience more if you wanted to chat.
Good luck, I would be extremely surprised if it's all you - if you've worked in the role ok before you can do it, there may be areas you need to brush up on, but a manager's role is to build you up and encourage you to shine, not to nitpick and undermine you. It's surprising how quickly confidence can be eroded by bad managers, especially when someone is conscientious.
I'd also suggest speaking to an employment lawyer - if it is the case that there's a clash or even if you just indicate that things aren't working out for you and it's partly down to management style which is making you unhappy, then it is likely they'd be willing to pay you off (at least notice pay plus a bit more) which would buy you time to find something new whilst preserving your dignity.
Wishing you well.