Was the filly in heat?

I am not a braggar but have years of experience fixing problem horses...Many problems like yours is a symptom to a problem but the problem is disrespect to you.This is very common when the horse wants to be in charge.Seperateing them will do little good unless the core problem is fixed...disrespect. I demand respect from a horse and it has to be consistent but not cruel.The horse actually makes the decision of how severe the correction is. In the long run you may be better off getting another horse that understands the human pecking order or lacking time get a professional trainer that understands the difference between a symptom and the actual problem and how to correct the problem....Sadly many so called trainers dont understand the magnificent horse.

for RIDERS WHO LIKE CROWDS we have herd bound horses for RIDERS WHO LIKE THE UNEXPECTED we have horses that bolt know where to draw the line...keep us posted

for SLOW RIDERS we have slow horses for FAST RIDERS we have fast horses for RIDERS WHO HAVE NEVER RODE we have horses who have never been rode

Maybe try something old school...? They "feel comfortabe" together, therefore then create the reverse situation where the problem horse is made "uncomfortable" in the company of it's barn buddy. Clinton Anderson suggests to keeping their feet moving. The subject animal is rewarded for the correct behavior by allowing it rest. Try to apply the same idea in other ways. Try tying her visually isolated from her barn buddy, but still on the property. When she quits fussing and relaxes reward her with a favorite treat or rub. Repeat frequently. Physically separating them is a given. Another suggestion would be to tack up, but pony her away from the barn buddy. Go all about the property and down the road. Repeating this process might (eventually) get the point across. Of course you did this step when breaking her to saddle, but it might be worth revisiting. Good luck. There is never a one size-fits-all fix when it comes to equines. Just try to be safe. Qtrpony aka Phoenix

Darn, that site doesn't come up for me goodhorse

Actually, my horse trusts me just fine, or we would never have made it through the parade 4th of July with all of the obstacles we had to face there. It's not a trust issue it's a buddy issue, that we are taking care of by separating the 2 of them. I was just hoping it didn't have to be a more permanent thing, because I was really wanting to get her back up her to the house, if possible.

My horse was both barn and buddy sour...... I have only had her since January, and I notice that the more ground work I do, the more she is willing to venture out. Sometimes she lifts her head high and trembles a bit, and I find that I often have to give her some light aid to gain her attention: a stop, back up, jiggle a rein, go in a circle, etc. But the Clinton Anderson exercise looks kind of fun too if you have a willing partner! Lmk how it goes!

This strategy by Clinton Anderson looks promising: https://www.youtube.com/watch?v=v-q9ffdaGYQ

Little... you're probly gonna have this issue, period. Seems you're on the other end of the stick in age, but for the same reason. They feel safer together. Herd animals, you know. I'm wondering how well they would do in a group, and got separated by the group?? Let them learn they are safe without the other. And do this often. (??) Hopefully, they will figure it out pretty quick, and not make this a lifetime problem. I'm thinking you might want to consult a pro, before they get "set in their ways". May well be worth the investment.