My mom always gets on my case for the way I say things.
Let me see if I can rephrase to be more clearly understood.
When you feel bad for doing what your vet instructed you to do, you don't do yourself, (or your horse) any favors, and in fact could make the situation much worse by letting your emotions get in the way of what is best for your horse. If you were told not to work the horse, then you can't work the horse, plain and simple. You are doing what is best for the horse, and if it makes her sad, its a bummer, but you should try to feel good about doing the best you can for your horse.
Now, if you're ignoring your horse because she's ill (which I'm not saying you are, just giving an example), then
you are a bad horse owner. In my mind, there is a big difference between 'not riding' and 'ignoring'. You had said that you felt like the worst horse owner ever, and this was meant to show what a bad horse owner might do.
In no way did I mean to imply that you're a bad horse owner, and I'm sorry that my post made you feel worse. It was meant to impart a little perspective on your very emotional situation.