Well, I wouldn't tell them all "where to go," (because I don't DO that sort of thing

), but I DO think you need to change your perspective. Personally, I wouldn't respond to these folks at all, I'd just say, "Okay, thanks for your opinions" and move on.
WHY on earth are you letting a bunch of strangers on the internet affect your emotional state, AT ALL? Don't take it personally--just realize that some people have one opinion, some an extreme opposite opinion…and some don't have the intelligence or sensibilities to craft an opinion so they are left with ridiculous responses like "they're horrible."
So, just dismiss the whole thing and move on. If nothing else, you've learned a valuable lesson. Don't expect strangers on the internet to give you the emotional boost you need to keep going.
HOWEVER: You *could* find some value by going beyond the harsh words they chose to seeing the essence of what they are saying, which is simply, based on the photos you've shown, you're really not ready to even START the discussion of "how much would you pay?"
Yes, I know you were not suggesting that YOUR photos are ready for that. But, WHY are you even pursuing that direction right now? Are you saying that if there isn't enough of a market to sell horse photos, you won't bother to learn how to do it well?
Look, you love horses. And you like photography; at least, you think you do--enough that you were willing to divest yourself of your horses. So:
Step One: Start learning everything you can about how to improve your photos, in general. Learn about proper exposure, good composition techniques, etc. You stated that you knew your photos weren't that well composed and had exposure issues, but then questioned whether they were actually "horrible." (And, for the record, no, they aren't HORRIBLE--but there IS vast room for improvement). Exposure issues and composition issues will KILL the effectiveness of a photo, period. So don't worry about whether it's horrible, or just bad. Just focus on how to improve.
Step Two: Start learning everything you can about taking good HORSE pictures. As Derrel mentioned, that can be tricky. Getting a good picture of a black horse, or getting a nice photo of a white horse, showing good definition and not blowing out any of the white areas, is something that can really take a lot of time to get proficient at.
I do a lot of bird photography. When I started doing it, about three years ago, I was TERRIBLE. Horrible. Truly. AND--I KNEW how to work a camera. I could get decent photos. But bird photography was a whole 'nother thing! It took a long time to get to where my pictures were anything someone besides my mommy and my sister could be impressed by, and the truth is, I'm still not where I want to be. Practice, Practice, Practice.
Step Three: Don't even WORRY about how to make money at it right now. Let that come AFTER you have gotten really good. By that time, you'll have a better idea of what direction to head in marketing yourself, or whether you really even want to do that.
Let tomorrow worry about itself; today, just concentrate on the issues at hand, which is improving your skills.
As a side note: I have three nieces who are very "into" horses. All of them rode in shows constantly when they were younger, and two of them still keep horses today (they are all in their 30s now and either married, or about to be married). Based on my experience with them and their mom, I'd say you CAN make some money with really well-done horse photography, but my guess is you'd do better selling "horse shoots" like people shoots. Doing a session with someone's beloved horse, and providing them with pictures worth framing, rather than taking pictures of horses they are selling. OR, photos at horse shows, session with the horse, the rider and their ribbons, action shots, etc.
In other words, my guess is that people will pay more money for good photos of something they are emotionally connected to, than they will for something they are trying to sell.