If she is happy and has good quality of life, you are happy with her and are able to do all you want with her. Then I think it is no different than any other horse with a health problem like an ulcer or arthritis. You have to medicate some of them daily as well. No one can tell you whats right or wrong as far as putting an animal down. Only you and your vet know that answer.
For most people it comes down to a matter of money. If you have the money to keep her, then go for it.
"Until one has loved an animal, part of one's soul remains unawakened..."
- Anatole France