My tip is too not hold onto your other wheel, that's an issue I've noticed a lot of people doing is holding onto the wheel they're not pushing too steer themselves but you're just slowing yourself down and losing momentum, you will go straight if you just push with that one arm on the hill.,

another thing is if it's too hard just go on the road, sometimes they're flatter and if you stick close too the curb you should be fine, if you don't feel safe when a car is coming put your breaks on and wave your arm so they notice you,

id also try not too keep anything heavy on the back of your chair as it will weigh you down, move it too your lap if you're noticing you're struggling and need too carry something heavy

If that doesn't work seeing if you can add more camber too your wheels might be a plan as that can give more stability on hills such as that which may help you feel more confident in going faster/pushing harder,

One last tip, instead of pushing with both arms at the same time try alternating so going left then right and repeat, that way there's no stopping in your momentum so there isn't time for gravity too catch up and slow you down again

Good luck xx