What difficulty are you playing at? How many provinces do you control? I did not encounter the problems you describe on normal but I wonder if you have become so dominant that the other factions are turning against you?

What I did as Oda on normal was get to a certain size - around 12 provinces or so, IIRC - and then pause. I was the biggest dog in the pack but not so close to 20 provinces and realm divide that I feared the other factions would unite against me. I used those fat years to build up a big war chest, so that when almost everyone did turn against me, I could cope economically.

Also, I had one ally with whom I maximised relations (+200?) - e.g. by marriage. They stayed sweet until well into realm divide.

Once the "total war" end game starts, I found armies became not so expensive whereas in the fat years I could only afford very modest forces. I think this is because once you start gobbling up provinces, your lands and hence taxes increase at a much faster rate than you need to increase your military. In the mid game, I probably only had about 2 full stack armies, heavy in ashigaru. By the end-game, I had about 4-5 with many samurai.