The horse I have been riding for my past few lessons is being quite difficult when I have a lesson with other people. He hates being away from the other horses, when I am at the back of the group and try to slow him down so we are spaced out, he is really resistant and tense, and when he is slow as soon as I relax a bit he speeds up again. Sometimes my instructor asks me to do a small circle away to space us out, and it is really difficult, when I use my legs he ignores that and when I use my reins he will just turn his head. Eventually he will do it but it takes a minute or so. Whenever I have to do something by myself he will act up, throw his head around and keep wanting to be back with the others. Once he just decided he wanted to go over to them and just ignored what I was saying.
I don't want to be one of those riders who blames everything on the horse, so is this my riding that is causing this, and what can I do to prevent him from acting like this? It's never happened with any other horse I've ridden.