I know I'm never to let the pressure off if he doesn't do the correct thing, but then I'm left standing with a horse nuzzled up against me, and I don't want to back off of him, so I'm stuck
This I find slightly confusing.
How I'm reading it is you're asking for back he's moving forward and you end up him standing right next to you.
What is your way of ensuring you can back up what you're asking? My suggestion would be to carry a crop whip something and if he keeps walking through you be prepared to give him a good whack or 10 on his chest.
What method are you using to ask him back?
The way I explain it to my students is think about when mum asks you to clean your room (yes I teach mostly kids) at first she asks, then she asks more firmly, then she tells, then she yell/grounds/takes toys away
Before You ask make sure you are focusing on where you want him to go stand your ground and keep looking behind him. Like with everything look where your going
The idea is you what to think about having a few 'phases' ask remind tell demand perhaps.
So if your asking off the chest you state just gently asking back for maybe 3 seconds, tapping the crop gently on his chest
Then if not response tap a little harder for 3 or 4 seconds
Then start to put a bit of bite 3 or four seconds
Then get harder if no response
Keep going until he backs up I'd be very surprised if he refused to back up but you will have to get tough!
If he walks forward get harder instantly and not just slightly harder, every step until he goes back you are not to move your feet.
The second he steps back stop asking give him a moment and ask again.
The idea is with repetition he will learn he has to move back if he does it sooner it's much nicer the longer he takes the harder you ask.
I suggest using a crop in this case because he will be resistant as he's been allowed to ignore you in the past and using crop means you can successfully back it up without hurting your hand.
Really hope that reads like it's supposed to