Congratulations! You're following your dreams and that is what life is about....setting goals and following our dreams.
When we are young no one tells us how hard getting older will be. Just, keep in mind, as a child you learned to walk by crawling and then cruising. One day you let go and stood for just a second before grabbing hold again. When you took those first tentative steps and fell, you get up and tried again. That's what learning to ride is.
The harm is not in the error but in not trying. So GO FOR IT!
Keep in mind a buck is often easier to sit than a spook sideways. Congratulations, you know your body will follow the shy
Ride the stout horse with lots of bone -your weight will be nothing to them.
Ride the steady eddy and leave the youngsters to the youngsters.
ALWAYS use a mounting block! FYI as a child in Bonn, Germany they never let us mount our horses. You were either given a leg up or you used a mounting block... if not, you didn't ride. Now, all my horses are taught to saddle up to the fence, gate, mounting block, trailer bumper or anything I have climbed up on so I can mount. I only mount from the ground if nothing else is available - not from need but to preserve their back. This "saddling up" has proven to be a great test for my horses readiness to behave under saddle. If they are not ready to ride (too full of themselves) they won't move over for me to get on. At that point I do a bit more ground work and try again.
You are a true inspiration so please continue to follow your dreams. If this instructor isn't the right one with a helpful attitude, find another. You can do this!