I am so sorry to hear that. It's a little different for me; I'm 51. I can't imagine facing these surgeries at your age. I rode professionally in my 20s and 30s, sometimes 10 or more horses a day. In my late thirties I changed careers and lifestyles, had a child and got an office job and settled for pleasure riding a couple times a week.
However, I now have horses at home and have been able to ride more regularly, so this is a blow. I have tried joint fluid replacement shots, braces, cortisone shots, lots of different meds, icing before and after. I've also had this knee scoped and cleaned up once; and a procedure called a lateral release along with scoping after that. Got back to riding really quickly after both the previous procedures. I've done lots of PT as part of the recovery from those procedures.
I'm also seriously considering mounting from the right -- it will absolutely break my brain to relearn the habit of 40 years, but my right knee is in much better shape. Both horses are sweet, sensible QHs - one has done cattle work so is probably used to being mounted from both sides.
I've known I was heading for partial or total for years, I just thought I could manage it a little longer without resorting to this invasive a surgery.
Recovering from this surgery and looking at fat, happy horses in my front field is going to be tough....