I've recently been through something fairly similar, and it was tough, there is no denying it...but it does come to an end eventually, and six months after the divorce I feel so, so much better, it was definitely worth it.
The poor concentration is normal, your body is in fight or flight mode so it doesn't bother with detail, and see what sleeping tablets - herbal or stronger - might help. If the kids are old enough for it to be safe, just knocking yourself out once a week so you get some proper rest really helps. And yes, sertraline is very useful for the anxiety spikes, just dulls everything down whilst you get through it.
Get yourself a good solicitor if you haven't already got one and don't bother with mediation again (your ex is just playing games and it's not going to work...plus who'd want to go back with someone who has threatened you as he has?).
The actual process of divorce helped me a lot...just by being a process that had to be followed; I had to make decisions when requested, but the solicitor was in charge of the process (I did a lot of the detail myself to keep fees low, but it still cost thousands. But again, definitely worth it.
Once the process is rolling, build in R&R time for yourself, a mix of with the kids/away from the kids if possible. I went to stay with supportive friends every couple of months, and allowed myself to be looked after (not something I'm used to, it's usually the other way round), and also booked a couple of weekend stays just by myself in a small hotel, just to decompress, read, sleep, have someone else make dinner. Physical exercise is good too, if you can find time/make yourself do it (I struggled with this but when I did it, it really helped), and build in a few treats along the way...rewards for each stage, or each month or whatever you need. Doesn't have to be huge or expensive, a manicure if that's your thing, or a massage, or a book or magazine. I also binge-watched a lot of undemanding TV, stuff that didn't need concentration but looked good and was interesting enough to hold my attention...I started with the entire Poldark catalogue on Netflix and went on from there!
Being overwhelmed is also normal, it is an overwhelming process, even if your ex isn't behaving like a dick (mine did too), so just acknowledge that's what's happening and take a step back, slow things down...there is very little you need to deal with instantly, and when there is your solicitor will let you know. Rest as much as the kids will allow, farm them out to friends or family so you can have a day to yourself on a regular basis, at first to lick your wounds and then, gradually, to start to value as "me time".
Your ex will only be able to delay each stage so much, eventually he will risk being charged with contempt of court (we were heading for this, and I had to threaten that if he didn't fill the paperwork in, I'd be getting the judge to make decisions and he'd be stuck with them). Also, don't sweat the small stuff - my solicitor was initially very concerned about the lack of financial detail from my ex, but (a) I knew what there was would be more-or-less accurate and (b) I got to a point where I just wanted shot of him (he was abusive) and I didn't care whether he won the Lottery, it became much more important to be free.
Best of luck dear @EntirelyFedUp and happy to DM if that would help - letting stuff out rather than ruminating on it (esp as you can't offload to the kids) is a good thing. xx