To be the fair, the balance in the early stages as an independent kingdom with no vassals is stupidly difficult. The AI marshal will inevitably come around to 10 versus 1 you in odds. The politics/diplomacy part of this game is something that definitely could be massively improved for Bannerlord...
The only way to succeed lies in preparation; cozy up with a faction and get assigned a lot of fiefs that you will keep when betraying them. This is in addition to the usual maximization of char stats, enterprises, relations, honour, RTR, etc. I personally never had the patience to do this in the unmodded game on highest difficulty.
So what I'm saying is: If you don't enjoy setting up an independent kingdom, it's perfectly normal and might be preferable to not bother. Or, you can follow the good advice posted in this thread by people who have done it, if you do decide it's worth your time and are willing to plan ahead. I've always ended up spending my time elsewhere in the game due to how frustrating this specific part can be.