When pain clinic gave me a TENS machine the nurse practitioner showed me where to put the pads for them to be most effective (& drew me a wee set of pad-placement diagrams for the various joints I was allowed to use TENS with). If you see a physio who works with TENS machines I'd expect they'd be able to do similar for you. Is there someone with you to make sure that if you fall asleep wearing the TENS machine you won't be there with it on for however long you sleep, btw?
It sounds as if you really REALLY need to see a different GP to get a referral back to gynae. Actually, are you not able to contact the gynae team who did your previous surger[y/ies] directly at all? Do you not have a review appointment booked? From what you've said you sound like someone who needs ongoing hospital care - if you're currently under a gynae team, get in touch & explain what's going on; hopefully you'll get an urgent review.
At the same time, though I know you'll have no inclination to, play the game & keep a food diary; everything you eat & drink (& the times at which you do so) plus any symptoms you experience. Being able to show your GP that symptoms aren't triggered by certain foods will help - or you might find that there is a more complex picture here & you need gastro input as well as gynae.
Whatever's happening, your pain at the moment is totally inadequately managed. Some people do have to just put up with being in pain, including episodes of pain that cause them to pass out/vomit/cry, because modern medicine can't fix everything & treating pain is complicated. But in your case there are still LOADS of potential options for getting your pain controlled, heaps of which your GP could prescribe, it's not a Red Drugs Red Tape situation, just... feck knows, frankly, other than possibly some raving [internalised] misogyny about periods not REALLY being any kind of an issue? The complete refusal to engage with the idea the pain might be due to endo is concerning as well as utterly baffling.
Ech. Am really sorry you're having such a grim time. Please advocate for yourself & request appropriate care.
(Oh & if you find heat helps with abdo pain, look out for the heat pads you stick to your clothes. No medication in them, they're [mostly] iron filings I think. I've only used the Cura-Heat ones but am sure there are loads out there. You can get "period pain" ones but I usually just use normal ones & sick them to a vest so I can cover exactly where I most need to.)