If it were me, lessons would not be the right course or action. After all, like AK said, you know this stuff. I would reccomend you go with other ppl, and if you don't already, USE A WESTERN SADDLE. It has more support, and if you feel yourself slipping grab on the horn. I also reccomend a very flat trail, steady horse. If you only own one, you can use a friends. But, maybe if you think: 'I have fallen before. The last one was worse, but I'm okay. It is all part of horsemanship' it can work.
Hope it helps!