We also tried out the Cavesson while longeing and she wasn't quite sure what to make of this strange new halter but it went very well, I think it's better for asking for forward impulsion than the Parelli halter, although the Parelli is definitely better for control. It's a little large since it was originally Virgina's (the proud owner of Coco, a beautiful but enormous Menorquín mare) but I think I'll be able to snug it up around the nosepiece with old socks or something like that.
My husband and mother in law tagged along and were both surprised to see that Starbuck was so happy to see me and obedient inside her paddock (basic walk, stop and back commands). They also took some photos and even a video which I'll upload as soon as I get them.
In the meanwhile I've found some videos on Youtube of people longeing with some of the same problems Starbuck and I have. I see so many videos of horses longeing perfectly that I tend to feel like an idiot, it's good to see that we're not the only ones struggling a little. Here you go: