What you're experiencing is muscle failure.
I think you need to drop your stirrups. In some angles it looks like you are getting sickled ankles (rolling away from the horse at the ankle) which is going to cause all sorts of issues - your weight distribution won't be level across your foot but more focused on your big toe, your going to be balancing off that teeny point in your foot, it's going to wear out your ankles and knees, etc. Your leg also looks like it needs to come back a bit more under you to have head/hip/heel alignment.
* I'm often reading and posting from mobile and Siri loves to make a mockery of the English language.