Entry in progress—B.P.
 
“Fencing takes two lifetimes to master” is a similar saying.
     
Wikipedia: Dressage
Dressage (/ˈdrɛsɑːʒ/ or /drɨˈsɑːʒ/; a French term, most commonly translated to mean “training”) is a competitive equestrian sport, defined by the International Equestrian Federation as “the highest expression of horse training”, where “horse and rider are expected to perform from memory a series of predetermined movements.” Competitions are held at all levels from amateur to the World Equestrian Games. Its fundamental purpose is to develop, through standardized progressive training methods, a horse’s natural athletic ability and willingness to perform, thereby maximizing its potential as a riding horse. At the peak of a dressage horse’s gymnastic development, the horse will respond smoothly to a skilled rider’s minimal aids. The rider will be relaxed and appear effort-free while the horse willingly performs the requested movement. Dressage is occasionally referred to as “Horse Ballet.”
 
Google Books
Chronicle of the Horse
Volume 53, Issues 40-52
1990
Pg. 66:
“He will know and understand the saying, ‘It takes two lifetimes to learn dressage.’”
 
Eventing Nation
Dressage for a Year: Behind the Scenes with Arden & Whisper
By Kate Samuels on Jul 30, 2013 9:00 am
(...)
“Dressage takes two lifetimes to master; there’s always something new to figure out. It’s so incredibly intellectual, whereas eventing is more gut reactions and quick thinking,” Arden said.
   
Google Books
Dressage Solutions:
A Rider’s Guide
By Arthur Kottas-Heldenberg with Andrew Fitzpatrick
North Pomfret, VT: Trafalgar Square
2014
Pg. 9:
It has been said that it takes two lifetimes to learn how to ride; Dressage Solutions is offered as a guide along the way.