1/50sec at 300mm is likely having a bigger softening effect than the ISO, unless you were shooting on a tripod (and even then the horses motions will be a little blurred). I'd have pushed the ISO higher to get a faster shutter speed ideally, that would help a lot in countering general blur.
The greens are crazy, I like 'em. Was it Fuji film that used to look like this?
In general, I like this. It's simple and straightforward, it's not trying to be great art. It's a horse, and a beautiful horse at that, in a beautiful setting. It's a little soft. Who cares? Sharpness is overrated.
Aye I know the feeling - however I've recently decided that one of the most important things with photography is to not be afraid of the ISO. In general once the aperture is as wide as you can go (keeping depth of field in mind of course) and the shutter speed isn't fast enough then its just best to raise the ISO without any worry. Yes the noise will be stronger, but its something one can deal with as opposed to blur.
A happy medium is ideal ,but so many times its just not an option. Also I try to stick to full ISO values as opposed to the in-between ones (there is some debate that inbetween full stop ISOs are not true ISOs but where the camera under/over exposes and then adjust the brightness to compensate - assuming it also comes with some noise reduction).