with all this talk of bitless riding...

If the horse is OK when he is just standing tacked up and only reacts when he is being worked then I would look at the person on the other end of the reins.
 
To throw in food for thought, I've had my horse for 20 years since she was 4. When we were younger we went through all sorts of bits too, needing a running martingale and flash for full control (she'd get over excited and bomb off) and settled on a kimblewick for jumping/xc. Ended up happy hacking for the last 10 years ditching the flash but still in running martingale and in simple snaffle which she tolerated (mainly due to hacking on a long rein cowboy style!). But she was still grabbing at the reins and always remained 'mouthy'. To add context, she's had dentist/vets visits every six months for as long as I've had her, and I had an experienced trainer. Wolf teeth have come out along the way too, etc etc. 6 months ago (at the ripe old age of 24) I tried her in a cross under bitless bridle and never looked back. Gone are all the issues, martingale is ditched, and she's a joy to ride and seems to love it. Not saying it will be right for everyone - no doubt it won't - but for my little mare it was the perfect solution and I only wish I'd done it years and years ago! Good luck :)
 
I come in from the other end so to speak. My Arabian gelding had been my endurance horse, so was ridden bitless for years. Then he had a career change to dressage, so had to be ridden in a bit.

He had a whole lot to say about the deal, being 13 years old at the time. And an Arabian - they never pass up an opportunity to tell you what they think.

So it was partly my hands - got sick of people telling me to use a flash. But also bit shape, action and thickness had a lot to do with it. We ended up in a Sprenger Ultra something - D ring, double jointed and has a lovely curve to it so it sits in the mouth really nicely.

While I don't agree with change just for the sake of it, I was impressed with the choices that are available now. Back in the day you had the choice of a mullen mouth or jointed snaffle, or moving into a stronger bit. Initially I found the choice bewildering, but am pleased that it was there, it made for a happier horse, and yes, my hands improved.
 
There are tonnes of bitless bridle options out there (I've just been researching for my boy who has dental issues) - and I know you can BS in a bitless bridle, and I believe (please correct me if I'm wrong) you can XC in a bitless as well. Its only dressage where you have to ride with a bit.

So you could do 2 of your 3 phases with the bitless bridle and then only use the bit for the DR phase. You can hire the Orbitless bridle for £15 for 2 weeks to give it a trial run, and the Orbitless sounds pretty good in the sense you can alter the placement of the reins to give you different amounts of pressure depending on how much control you need. So it can work as a simple sidepull (only putting pressure on the nose) right the way through to working like the cross under bitless bridles that apply pressure to the poll as well as nose.

I'm a big dressage fan so its hard for me to accept going bitless, but having rode my boy in a headcollar and seeing how wonderfully well he did its made me realise you can do a heck of a lot without a bit. So my plan is to ride in the bitless 70% of the time to relieve the pressure on his mouth (he's got huge gaps between his teeth which have to be packed, these packings fall out often and cause pain) and then 30% of the time we will do some work with the bit to prepare for competitions but only when his mouth is good.

Appreciate its a bit of a different scenario for you in that there are no dental problems, but there shouldnt be any reason why you cant school most of the time (and jump, XC, hack) in the bitless then occasionally do some schooling with the bit in preparation for competitions.

If he isnt resistant to tacking up (my boy is very resistant to even putting the bridle on when he is sore) then it suggests it isnt a pain thing, and just needs some more schooling. Definitely worth trying out a few different trainers to see if they can add any value and give any different suggestions, but it sounds like (if it is just him being very heavy in your hands) then it can be resolved with some patience and careful training.

But as I said before, I dont see why you cant give a bitless a try, do your jumping, XC and hacking in the bitless - and see how schooling on the flat goes bitless. I posted a question the other day about bitless and how people who compete DR cope with going between the bit and bitless, and most people said its no problem at all. They use the bitless most of the time but do keep varying what they use to make sure the horse is used to both bitless and the bit, so when it comes to a competition it is no issue to switch back to using the bit.

Definitely worth giving bitless a try, and worst case scenario is you can event and do the SJ and XC bitless, then have to go back to the bit for DR and he is just a bit heavy for your test. It shouldnt affect your score *too much* providing it still looks (to the judge) round and not on the forehand. Even if it feels heavy to you it might not look so bad to the judge and your marks wont be affected too much. But hopefully as he gets older, and as you get more training together (try using 2 or 3 trainers regularly, I know too many people that stick to 1 trainer for DR but reality is there are a lot of great trainers out there, all with slightly different approaches but you can take elements of each approach and make it into your own approach that works best for your horse). I have 3 trainers - one pro DR rider who trains with the Eilbergs, YO who is great with problem horses and takes a more wholistic approach across all disciplines, plus a 60+ year old GP rider who trained at the Spanish High School (he only visits the UK 4 times a year). My Spanish trainer always says he's developed his approach to training because he has trained in so many countries with so many different people - its not a modern or classical approach, he takes whatever bits he feels most relevant and applies that to his horses.
 
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