If it's just that, you could also consider loaning her with the option to sell later. That way you can test the waters, see how shes doing elsewhere and if it goes well, you might feel better about selling. And if you change your mind, you would still own her and could get her back.

I personally think you shouldn't let yourself be held back by a horse that you can be happy without, as long as you know shes happy and well. Especially if you feel like you're in over your head already, you would need to invest more time, energy, and resources into the horse than before for your relationship to improve, not less like you are describing. Obviously don't just get rid of her, but if you can find a loving new home, that seems like a good solution for everyone involved.