Rider problem with new horse

[taiyou_no_ao]

Hello everyone, I'm a long time lurker, first time poster. Recently my boyfriend and I started leasing a new horse, a 16 year old Selle Français named Hermes. He spent a lifetime of showjumping all around Europe before our barn acquired him from a boarder who needed to sell him. So for the past year he was mounted by a trainer sometimes but mostly used just as a school horse for lessons, which made him bored and lazy. We have only known this horse for about a month, and been leasing for 2 weeks, so we are still very much in the getting to know him phase. Neither of us are beginners and we've both done much showing and had our own horses in the past.  When we first started riding him everything was fine he was just a bit slow, but everyone was just saying it was because he was being used in lessons and he was bored so not putting in much energy, he is a jumper and was not really getting to jump that much. So we did different exercises with him to get him out of the arena and interested, such as taking him to the country field and gallop track and the change of scenery made him much more forward. But now we have a new problem. Since he has been ridden by not so advanced riders lately his head set is a mess and he walks/trots/canters all with the flag head. So we're trying to reset his head and get him round and working with the back again. Of course he is fighting it. With my boyfriend (who I guess is the more "severe" one) he rounds his head with no problem at all gaits. With me I can get it at walk, but at trot and canter he does it for a few strides and then puts his head back up and resists. But ever since we started working with him on the head, he developed a new problem with just me. Not with my boyfriend or anyone else or trainers at the barn. When I ask the horse to trot he thinks I'm asking for canter and starts to canter or do a weird trot/canter combo. And the problem is getting worse each day, and we keep trying many different ways to fix it and the horse resists more and more. He is even starting to bite the bit and bolt forward when I try to stop the unwanted canter (he did this today for the first time)... I have tried to stop the canter and ask for walk before asking for trot again, I have tried to keep contact with the mouth while pushing him forward, I tried sitting deeply and back in the saddle, bending him strongly to the opposite side (so he has no canter balance), but he either will bolt forward or walk, or continue to canter with his head turned almost on my knee. When I first started with him he trotted fine, with no problem at all except it was a bit slow. This refusing to trot thing has just started within the last week when we started to work with the head. I really don't know what I changed.The trainers can't get on and "correct" him because he only does it with me, not even with my boyfriend. Its more of a "me" problem than a horse problem. And now the horse is resisting more and more harshly to me. At first he would just half canter the first few strides when I asked the trot but if I kept contact and continued to post he would eventually stop and just trot forward. But today I couldnt get him to trot at all. It was either walk, or canter. I am a fairly advanced rider and i've ridden many horses and this is the first time i've had such a problem, and he is a highly trained and experienced horse. But at the same time I have just recently come back to riding after an almost 4 years break. So i'm at a loss to fix it and there is really nothing I can do with the horse until I fix this trotting problem. He is a lovely horse who can jump ANYTHING and I want nothing more than to move past this problem and have fun with him.

Here is the cutie:
(links since I couldnt get any pics to upload)
http://i55.tinypic.com/p4p03.jpg
http://i56.tinypic.com/2lk4cqq.jpg

Side note: Anyone know anyway to avoid/remove the spur marks? He had them before we got him...