Training a horse to be alone

It is completely normal for a horse to become stressed when alone. Building her confidence when she has only a person for company takes time and patience. It sounds as though you have made progress but still have a little to work on. Your horse is still young and is finding her feet as a riding horse rather than a racehorse, so give her time and avoid putting her in situations that are too difficult.

On the days you cannot ride in company, you could work from the ground near the paddock so she can still see the other horses, or potter about with her just outside it. When the days are light there is plenty you can do outdoors. If she is settled with that, increase the distance from the herd in stages, but if she starts to get stressed, back off and shorten the distance again.

When she gets as stressed as you describe, her only focus is getting back to her herd. You do not need to feel like a bad owner in these moments. She is seeking the safety of her herd, and it is not about her not respecting you. Trying to correct her at those times will only add to the stress.

I hope this gives you a clearer picture, and good luck!