Can you get your surgeon or GP to refer you for physio after the op? If you explain your specific need (safer falling off horses!) they might be able to show you how to build your own springs and relax more. Permanently tense muscles that have been used to compensate for an injury can need some work themselves. Lived with someone who made remarkable progress on this after physio and their injury was 20yrs old!
Failing that you could always try these
bounce shoes