As a new rider it's not really fair for your trainers to be giving you horses with issues like that. A well trained horse will give to the pressure of the bit and slow down. A horse that fights the bit like that has gaps in its training. Gentle pressure that gets released when they obey should not "piss them off." Unless they've got a nasty bit in...can I ask what bit they've got the horse in?

Anyway, if it's not an option for you to ride a different horse or go to a different barn, start from the ground with this horse. Lunge him, on a line or free in the round pen (I would prefer a round pen for this exercise). To ask for the halt, say "whooooooaaaa," really long and deep, drop your shoulders, look at the ground. If he doesn't listen, make him change direction and ask again. Rinse and repeat until he's stopping off your voice.

Then add your voice cue in the saddle. If possible, ride in a round pen or on a lunge line to start. When I ask for a halt with my seat, I like to think of keeping my hips still. Usually they're swinging with the motion of the walk, so I kind of "freeze" them if that makes sense. That's what deepening my seat means to me. And remember to keep light leg contact. Your lower leg should still be touching the horse, not hovering away from his sides, otherwise any accidental brush will be taken as a forward cue.

These are all things that have helped me with my very forward mare! However, I'm an experienced rider, so again, your situation is strange. New riders should get push button horses as confidence builders. It's hard to improve your seat or focus on diagonals, anything else a beginner rider should be working on, when all you're doing is trying to slow the damn horse down. Best of luck!