Hi Lucy, I have been going through a similar situation with my youngster. I've had since she was born, she's now a 4 yr old, I backed her earlier this year with out a problem and was riding her out 2 to 3 times a week on short hacks with another pony. Our problem began when she started to nap, I have to pass the field where her pals are, the field has 2 gates and it runs the length of a long drive, and then a couple of hundred yards of the road I take. I won't go into great detail, but I got on one morning and she wouldn't leave the farm yard, she backed into and sat on my friends car, then backed into a cattle trailer and got her leg through the V of metal where the tow hook is. It was at that point I got off, got my long lines and drove her out on the ride I was going to go on.
Since then I've took a couple of steps back in what I've asked of her. Outrider gave me some excellent advice, he told me to get her obedient on the lunge. She has always lunged pretty well, but slow to respond, so I became more insistent, demanded that she moved up a gear immediately not 4 or 5 strides later. Did I have some fireworks, big bucks, stamping and grunting. I just let it run its course, didn't back off and kept quietly insisting, she settled after a while and worked well. I tried riding again and had the same response, she wouldn't walk on, backed up and bucked, I sat her out and she rode back to the yard, Thank the Lord! But it was at this point I knew I needed help.
I rang a good friend and told her what was happening, a couple of days later she came to help me. She brought with her, her western saddle. We tried it on and fitted, so we lunged her in it, she walked, trotted and then bucked like billy oh! but after about 5 minutes of objecting she went from a she-devil to a willing, forward going, young horse. I got on her and rode her back but still clipped on the lunge, it was like riding a different horse, before though she went forward it was at a snails pace, I'd never interfered much, just thought she'd find her stride as she became more confident. But the new walk was much more relaxed, forward going and swinging.
Since then we,ve done more of the same (but only 2 or 3 times, as my friend works and has loads of other commitments too), about 10 minutes lunging, then a short ride, my friend walks out with us, she hasn't put in one objection, she is now responsive and forward going (sometimes a little too fast, but she does listen to me).
Today we didn't lunge, I got straight on and we rode up the drive with my friend walking along side, she didn't even think about stopping at the gateways, though she did stick further out, but it was half hearted and she moved off my leg without even swishing her tail.
My advice is to get someone with experience to help you, plan what you are going to do, take her back to a level you can work that she's happy with, be firm but sympathetic and work out in advance how you'll deal with any problems that might crop up, I know some situations are impossible to fore-see, but you can engineer situations to work in your favour.
One thing my instructor drills into me is 'always think about what you are teaching your horse' if you're getting a bad response, take a step back, do something your horse is good at, and always end on a good note. Your horse sounds very like mine, young, is very well respected in the herd, and loves humans. Mine is very confident which is a good thing, but can backfire as if she decides she doesn't want to do something, she's not easily persuaded to change her mind.
I do sympathise, I've done alot of soul searching lately, doubting my ability, and my confidence has waivered. I know I have a way to go yet, but now I feel positive, and a strong sense of determination has kicked in.
Hope things improve
Lesley