Hi Lily,
How frustrating! It's hard to know what to recommend. Does Stella tend to act more anxious on trails than in the ring? (I know you've posted about her before, but I can't remember if you mentioned whether she is better in the ring or on trails)
If she is better in the ring, I would try to end all of your rides on a good note in the ring rather than going out on the trail after a ride in the ring. When you do go on trails, one thing you might try to get her to relax on trails is handwalking her. This is definitely not effective in a lot of situations, but I've found that sometimes it can help settle down a horse that is genuinely nervous and not just misbehaving.
You sound like you are doing a great job staying calm and encouraging her to relax--it's great to hear because we all know how easy it is to get worked up about it and make things worse
So, what to do now? I think I would try to keep things simple and relaxed. Is she ok when you first get on? If so, then you might try a short ride (maybe 30 minutes) and stick to the basics and if she goes well then call it a day. Then, try gradually increasing your ride and the level of difficulty of what you ask her to do.
It's hard to give advice without knowing exactly what it is that's setting her off. It seems like today it was going on the trail, but from what I remember about your other posts, that's not always what gets her worked up. It seems like you do a good job of getting her to work when she's upset, it's just that you can't get her to relax on a long rein. If that's the case, then I would try to put her in a nice trot and then work on transitions from trot to canter and back down to trot. Then, I would try some trot-walk transitions. You could try only keeping her at the walk for a few strides and then going back to trot. Then, go back down to walk again but this time for a few more strides and gradually increase the number of walk strides you ask for between transitions.
Anyway, I hope that some of this helps, but it's really so hard to know without seeing her. So, I guess my last piece of advice would be to get a good trainer or knowledgable friend to watch you ride so you can get another opinion. These things can be so tough to figure out, sometimes having someone else's perspective can really help.
Good luck and keep up the good work with her
