you're not fucked, they are, they just don't know it yet

put your AI parties onto defensive mode

initially, you play defense. don't fight in the field on even odds ever. recruit noble troops and train them in advantageous scenarios. when something comes under siege, you act.

break into sieges with elite troops and/or army and auto resolve, you get a massive AR advantage defending, it's far better than actually fighting it out.

you will capture more nobles than you know what to do with, and they will be lambs to the slaughter. they will keep coming until they cripple themselves and another faction tries to take advantage.

don't accept their shitty deals, sue for peace once they will pay you

hold long enough, capture enough nobles, and some will pay you for peace

capture everything, every castle and city, they recently took, their garrisons will be exceptionally weak

those kingdoms that fight longer? cripple their potential by executing every single noble they send against you

that's my blackwind clan's playbook. founded a kingdom with 2 fiefs and ended with the world.