"DH will go when he has to. He is clueless though. He is on the spectrum and finds it stressful to degree."
How clueless is clueless? You say you don't really know that much about horses ( "I don't know what I'm looking at I just want her to stay on tbh" ) so what is it that you think DH needs to know to accompany her?
"I work an intense job with unsociable hours which is the only reason we can afford said pony DH feels he never see's me and younger child starting to become resentful and says things like "great so another day I don't see you because of x's hobby" as we head out for yet more jobs/yard work/day of competing whatever."
So, to afford the pony, you work "an intense job with unsociable hours" which means your husband and your children don't see much of you during the working week. Except, your daughter has you all to herself at the weekends because of the pony.
You have a work-life balance issue. And a life-life balance issue on top of that.
Would you prefer to have a less intense job with fewer / more sociable hours? Is that possible in your current job, to move down a gear / transfer internally / take a demotion / go part-time? Or just plain - find another job? Would your reduced income still meet the bills, barring the pony? That would address the work-life balance issue, so you could actually spend more time with your family.
Then, the life-life balance issue. Right now, the whole family is revolving around a pony. And you KNOW that's not fair. Only one of the four of you actually care about the pony, the other three are having their life distorted by its presence. That's not fair.
Your daughter doesn't have to give up riding completely ( "Daily I think about selling him and putting her back in a riding school" ) but she does have to give up being in control of everyone else's life. IT WILL NOT "DESTROY HER". She'll still get to ride, but you won't be dancing attendance and your husband and son will get to spend some time with you, instead of being relegated to the sidelines as at present.
"I feel pulled in all directions and I don't know how to fix it."
Yes you DO know. Sell the pony, put your daughter in riding school, her dad takes her there half the time, spend the time freed up with your son and husband, not just your daughter. Consider whether you really want to do the intense job with unsociable hours, if you do, fine, if you don't, change jobs. You do know what to do.
It might also be worth considering how you let this situation arise. It didn't just spring full blown from zero, it crept up on you because you took your eye off the fairness ball. Is your daughter the squeaky wheel that gets the oil? Did you feel guilty so bought the pony? Did you fancy the prestige? Ponder it a bit, decide for yourself what your weaknesses were to get into this mess, and work on them.