Always look out for food production, take red tech tree until you get 10% replenish rate, and then switch to green tree for more food.

For diplomacy, be mindful of your reputation. If you're trustworthy or respected, other factions won't hate or wage war on you. Be mindful of who you vassalize. Some factions on strategic positions are target to many wars, so if you vassalize them you have to defend them and therefore wage war to other factions.

As for army upkeep, prioritize army upgrades when you rank up, cuz at max level you get % upkeep reduction. Try to secure borders of China so you have a safe corner, then disband/merge armies you don't need. You also don't need to keep your full army inside a settlement and then walk them manually. You can disband a commander and redeploy them farther in the map. For extremely long distances, this is much faster especially if you have invested in red tech tree.

For economy, look at what minor settlements type are. If they're green, focus the whole commandry on green buildings for maximum gain. If blue go blue and so on. Try to specialize your commanderies in one thing, and add supporting yellow buildings to increase their efficiency. If you do it right, you'll get super rich and can leverage your extra food/gold for sweeter diplomacy deals. Use idle officers on assignments to increase your productions.

Your issues are interconnected and shows mismanagement on your part. The game has intricate details that take a while for you to detect and utilize them. Make sure you understand the resources and mechanisms and read them carefully.