Horse anxiety problems

Ive had Dutch for nearly a year now and when I got her she hadn't done a thing apart from a local dressage day.
She's perfect in her paddock and the bottom part of pony club but not the back (where the jumps are) she just loses the plot and its a mission to get her to calm down again. I wonder if she can see something I can't? I know it sounds weird but a lot of horses I know act up at the back on pony club.

And also when I take her out, If I take her out and no other horse is there she is brillant and doesnt put a foot wrong but if there is another horse she just goes nutty and wont stand to be tacked up or anything its just a hassle,
I recently had to get my old mare put down and Dutch was very attached so could this could be triggering it?


Any help is appreciated :)
Thanks

I'm not sure what "the bottom part of pony club is." Could you clarify?

It could have something to do with her friend being gone, but I'd only say this if she's being turned out alone now that the old mare is gone. Some horses are more herd-bound than others naturally, but if she's alone most of the time, I'm not surprised if she's going a little belibierish whenever she sees another horse. Horses are very social creatures.