Good for you!
I am just getting back into riding after a 20+ year break. Four kids, PCOS, fibromyalgia, low thyroid and valley fever, I was up to 255 pounds at one point. I managed to get down to 215 but gained 25 back after I went on an antidepressant when my mom died.
I'm back down to 228 (thanks to two weeks of caring for my new horse :) ) and I have already experienced fat discrimination. The other day a vet told me that I'd be crazy to ride my 15.2hh, 1060# paint horse. Granted he's old, a little lame above a walk and needs to gain weight and muscle, but the general consensus has been that it'd be safe to ride him gently at a slow walk and gradually work him up to more and more exercise.
To be honest, I haven't mounted him yet because I'm afraid people will be like, "Oh my G-d, she's going to crush him!"
Trying to get up my confidence! You look great on your horse and at this point I'd be very happy to be your weight and level of fitness!