It's actually quite doable to win against the AI while being badly outnumbered if you're doing it right. I fought HRE, Russia and France as The Knights (wasn't weak obviously, owned almost all of the Mare Nostrum and Africa) and won, being outnumbered about 4-1 to 6-1 in several wars. It should be easier for you, since you're playing Prussia. You simply have to sit in 100% (or close to) mountain provinces with a small but not to small stack, let yourself be attacked and keep reinforcing until you win. Don't hunt them down, just consolidate, set up the trap again and repeat until you've killed their manpower (yes, that is possible even against HRE, France and Russia) and drove their WE up. You can try to siege provinces close to your trap with small merc stacks, but avoid fighting if there's a reasonable chance you'll miss the -3 terrain. It's going to be stressful and I would probably prefer to restart in your case, but it should be possible.
If you've picked decent ideas for your military (like Offensive, Defensive, Quality, Innovative) I'd wager you should be able to beat about 6-1 odds in mountains, I managed to do 4-1 without the Prussian supersoldiers consistently. You should definitely have some nice cash backup before you start though, you'll need a lot of mercs. You should also check the rivals of everybody you want to peace out separately, get military access with those and offer "annual treaties" to get them to peace out. Combined with enough money that should give you the chance to peace out the ones you don't want to take anything from pretty fast. If I were in your shoes I'd probably go for a very early peace (depending how it's going) and immediately break truce with the HRE, that way you'll have them separated and without allies to go for nasty 100% peace deal (if there's no coalition). And you should definitely try to make Austria release small nations instead of taking things directly and diplo-vassalize them yourself.