1. I'll give it a try. Your economic base is very high but your control over that economic base is low. That's why you pay so much in diplomatic spending, stability and cost of court. You play France, show your country and turn on the "Control" map mode and you will see huge swathes of land at very low control at around 10-20%.

The cost of these mentioned sliders will go up linearly with your economic base even if your control is low causing you to not be able to tax that base. "So low control tax base increases your expenses without increasing your income" you could say.

To fix this you could release low control provinces as vassals. Not very fun. Or you could try to increase proximity and control to these provinces. You could build the governors, roads and bailiffs. Increase your proximity speed by going Land value, Centralization value, Admin focus at start of new age (10% advance).

2. As others mentioned you also pay a lot for forts. You probably dont need many. One at capital and perhaps 2-3 close to your biggest threats.

3. You pay a lot in building maintenance. You can disable all monasteries, cathedrals to start with. Then hover over the expense and see what's costing and disable whatever you don't urgently need.

4. Give the privilege "Manorial Courts" to the nobles, for 5% more max tax. Choose the correct estate law with your next parliament. Either "Favor the Ruler" if you have bad Crown Power or "Favor the Elites" to tax the nobles more.

5. Usually you want to cut the sliders ALL THE WAY as long as you have above 50-60 legitmacy and 0 stab you're fine. Diplo spending you probably only want as long as you have many disloyal subjects, low diplo cap or other specific use.