I had the same problem with Owen. After a year of only a bit of lame walk, his whole body, lungs and mind was shut.
(this might sound idiotic but hey, it works for us)
I then put him in the outside arena, take a piece of rope and ask him to walk around me.
I do all sorts of energetic stupid things and make noises, to make him 'laugh' and feel funny and playful.
When he starts to trot, I am so happy and so proud I need to call the newspaper!!!
And then when he starts to canter... drop all, people of the world!!! Let CNN come over now!
The most wonderful thing is happening!
Yes, he is overstressing himself at some point, but, one has to, to build stamina.
When his eyes are starting to pop a bit, I ask him to stop... after all, how much of a wonder can one person handle a day? My dear heart...
Then I go on to Ralph in Owen's presense about an hour to express the miracle, the beauty, the machoness, the mystique... of Owen's canter and trot.
And Ralph looks at him in awe... wow, I can not do that you know!
Owen feels like a King... and every King canters a lot...
He again enjoys his canter a lot and it gets very easy now. You see his belly muscles lift, his back lift and his hind quarters grow muscle.
He can rear again!
For Evita's problem... you need canter for sure.