So I'm on my laptop, which means this will probably be a long reply. I apologize now.
So first off, really work on getting the feel of your horse pushing into the bridle (not pulling, you but coming from behind and pushing right up into the bridle). If your horse can't give you that feel in his jumping tack he has no business out on the cross country course.
What it really sounds like is that he's lost his confidence somewhere along the way. So he really needs to be taken back to basics out there. It isn't fun but a horse who is that ready to run out, is like that for a reason. Take him back to the little stuff, and to it until he can do it in his sleep. I mean the maiden stuff that he can step over in his sleep, if he's still giving you problems with that, maybe he's just not cut out to be an eventer.
That might just be the cold hard truth. Not every horse has the guts to make it cross country, and it really isn't fun (for you or the horse) riding a horse that you have to chase over every jump. They get more nervous and stressed out and scared, and you develop agressive defensive techniques that shouldn't be necessary.
So although I'd like to say that every problem has a cure, you can try to help him out but if he's still being problematic, he might just have to stick to the jumper ring. Better to let him shine at what he loves than to force him to do what he's afraid of.