Oh dear! I would suggest a new GP, but actually you probably would not do any better. it also sound as if ageist,at 54! I am really old and very ill, but have not been written off by hospitals, docs etc! GP`s are not good on thyroid or diagnosis, unless very lucky. Common , does not mean simple!
Also make sure you have had the other tests needed with thyroid, especially vit D, , hormonal, corrected calcium before treating with D Re test after 3 months for them both. B12 + foliates, needs to be high in range, Diabetes, these are both autoimmune hormonal, annual tests for all . Also iron/ferritin, needs to be clearly in range, simple tests all blood..
I would also keep an eye on your thyroid tests, yourself, if GP will not do enough, on line.