Hi there adarkhorse I really enjoyed reading your post. I am so glad that you were able to gird your loins and get on the horse. Please don't give up before you have started, if you have a true passion for horses and horse riding - and it seems like you do, then get to it!
However and on the other hand, I do think you would be happier and more confident if you lost weight. I think horse riding could be just the catalyst you need for positive change. Please do not think that I am suggesting that you stay off a horse until you lose weight, I really aren't. I am speaking from my own experience as a reformed fatty. I weighed 115kg and am now 70kg and I have to say that my life is whole different experience.
Horse riding is a whole new world - for both my horse and I. There are so many things that are different with my horse, I mount from the ground on both sides for a start. We have started jumping (for a 3/4 draft my Phoenix is a VERY handy jumper
) and most important of all my Knees Don't Hurt Anymore when I ride!! It had got to the stage when I was heavier that any longer than 2 hours in the saddle just killed my knees. My theory is that without all the extra inches of flab forcing my legs wider my knees are now in a more natural position and a lot of pressure has been relieved. So yay, no more pain!
Do think about losing weight from the perspective of health and quality of life. I would imagine that with the physical issues that you have the less mass your joints and ligaments have to carry the better. I think of it like this: imagine the thinnest friend that you have, picture her getting up every morning and strapping a 40kg pack on her back and then setting about her daily business, going to work, cleaning house, chasing kids. How long do you think it would be before the physical effects of carrying that pack everyday start to show? In back problems, knee and hip problems? Best solution? Take the pack off.
I really hope I don't sound smug or condescending because that is the last thing I want. I just think that for the sake of your happiness and future horse riding endeavours weight loss would be a very positive thing. I gotta say I am so much happier now than I ever was when I was over weight. I would love for everyone else to feel the same way.
I am so glad that you have found your way back to horses, let them be the thing that kick starts you into a whole new phase of your life.