Sounds like saddle doesn't fit (with you in it) - so a saddle fitter then a chiropractor is the way to go.
Until then if the front of the saddle is too low take a dish towel and fold it in half then take that folded part and fold it again so that you end up with 2 thickness of layers. Place the thickest part under the pommel so it raises the pommel up a bit. Gel pads are not very good - when you put pressure on a gel pad the gel is squeezed out so you loose the padding where you most need it.
In the interim try riding horse this way to get a better whoa. After you warm him up at the walk (bending, getting him off BOTH legs (leg yield), ... start trotting asking him to "arch" his neck but allowing him to place his head low if he wants. The minute you feel him try to pull on the reins, lean on your hands, and/or stiffen his neck and/or poll - HALT. Make sure when you ask for a halt your legs maintain a connection with his barrel so you get him to step underneath himself.
If at first he ignores halt - it won't be pretty but demand a halt (no matter what - hauling on his mouth is OK as you want him to understand when you ask for). When you ask for whoa he is to stop.... then pat him/good boy for halting. From halt go back into trot BUT don't stay in trot more than 5 steps before asking for halt again.
The reason you make the trot steps so few is because the longer he stays in the trot the heavier he will get on the reins (think freight train). This time halt should be easier - if not get VERY strong and ask for halt again.
Repeat MANY times (trot/halt/trot) with the idea that when you ask for the halt it SHOULD get easier.
Common reason the halt might not work:
1.) When you halt sit straight up and down - do NOT lean forward OR back.
2.) Push straight down (NOT forward) with stirrups when asking for halt.
3.) Lock elbows on your waist and brace entire body during halt to discourage forward movement.
4.) Keep legs against horses sides - this is to keep him in front of your legs and to keep him straight. If he can "bend" his body during the halt he can get out of doing the halt.
Think of your body position mimicing your "standing" in the stirrups - legs pointing straight down and body position the same as you walk - shoulders back, head looking forward, elbows on waist.
So when you can get a trot/halt without pulling his mouth super hard go back to the same exercise with a slightly different twist - just as he starts to halt soften your elbows and LIGHTLY squeeze you legs so he "hesitates" at trot but doesn't stop.
This is a Half-Halt and makes horse sit on his butt and off the reins. You should practice doing this MANY MANY times throughout the ride - before every corner, before you ask for a transition, etc... at every gait, before every jump (2-3 strides out). When he starts listening to the HH you'll be able to ditch strong bits and go back to a snaffle.
If your trainer never taught you this to teach you about the half halt you might consider taking some dressage lessons from a GOOD instructor. Dressage allows you to adjust any part of the horse at any time (when done correctly), even between jumps, etc. It makes you a better rider and the horse a better trained more responsive animal.
Hope this helps.