I first became unwell after my second child, that was back in 1973/4, up and down to the doctors for 2 years before he thought to test for underactive thyroid. I don't know what tests were done but I was put on thyroxine as soon as he had the results. He certainly didn't go by symptoms, just told me I was a young mum with two children and I could expect to be tired!
It's hard to believe that your GP had you on 800mcg Levo without testing, of I was tested in 1975 then they were certainly available when you were on 800mcg in 1984. There has been some serious neglect by your GP. And unbelievable that the hospital doctors thought you only needed Levo if you had antibodies!
Well, at last you now know and have had your own test done, hopefully your GP is willing to be educated!
I'm afraid I don't know enough about rT3 to offer any help, I've read differing things about it so have no idea what is the right thing to do. But it is obvious that 200mcg Levo was too much for you.
How are you feeling now on 125mcg? You really need to give 6 weeks between dose changes for it to take effect and see how you feel. 200mcg > 150 > 125 is a big drop on just 6 weeks since your test.
I think you would be best to ask for a referral to an endo who has good knowledge of thyroid. If you email email@example.com she can let you have the list of thyroid friendly endos. See who you can manage to travel to and ask members if they have any knowledge of them, good or bad.
With your Vit D, are you taking the co-factors K2-MK7 and magnesium. And a B Complex to go with the B12?