Sorry, I know nothing about horses. But, I just loved riding, feediafter ng and looking them.

[ModreB]

OK. When I was in Australia, I was about 14 stone, and managed to ride an ex race horse with no issues. It was the first time I rode, loved it. I came back to the UK and was told unless I could get below 13 stone, it would not be possible.

So, I've tried looking after them, mucking out, grooming, (including being Bitten)

I am now just under 13 stone, but, could I ride now?

Yes you can. A riding school near me weight limit is 21 stones!😯

I am sure you can ride. Not sure how you can ride race horses, but most riding schools should be able to accommodate a 13 stone rider easily. Personally I wouldn’t put a novice in an ex race horse anyway and you are probably at the limit for a TB. Especially as non riders tend to sit more heavily.
Personally I would do a few visits to local riding schools and suss out which seem to suit you best. Do you have a horsey friend you can take along with you?