I know very little about driving, I've only ever been a passenger who's been allowed to hold the reins a couple of times. One thing that has stuck in my head though is that there's even less margin for error driving than riding, and I should imagine with a pair that's even more so than a single. I would stop driving this pony until you can get some expert help out and they can make an assessment of the problem and help you sort it out.
As an example of why you don't try carrying on yourself my first pony was an ex driver. He'd had an accident - not his fault, a lorry clipped his cart - and bolted down a busy road until he failed to make a turn and ended up in a ditch with his cart on top of him. Thankfully no one was more than badly bruised and grazed and he had no lasting physical damage, but even in old age you couldn't put a martingale, breastplate or even neck strap on him, or a bit that in any way resembled a driving bit. If something goes wrong driving you have far less control and options than ridden, please don't risk yourself or take the chance of ruining your pony for driving for life, not when some professional help could stop the problem in its tracks.
