Sounds like there's a few issues. Some tips:
1. Always ensure you have a warlord leading every army, and focus purely on leadership and reign. It'll reduce the upkeep costs of *every* unit, and the warlord gets a lot of beneficial skills to province ruling and armies they lead.
2. Only ever give provinces to your warlords, and pay close attention to any skills they or their wives have which will affect the province. Never hand a province over to a sage or champion unless you absolutely have to (i.e. loyalty drop without one)
3. Champions, sages et al should be placed in the same armies as your warlord leaders, not lead armies of their own. Not only do they too benefit from the warlord's skills, but you can afford to focus these purely on fighting and magic. Also note that even without any particular skills, the mere presence of a hero in a unit will let them achieve victories over units which, on paper, should slaughter them. Although this applies to enemy heroes too ...
4. Only recruit the armies you need. You don't have to defend every town, stronghold or even province. Income is generated each winter, so as long as you can recapture the province by then it makes no difference who owns it the rest of the year; similarly, it's usually better to focus on wiping out one enemy entirely that defend against multiple fronts; losing a province in return for eliminating an entire kingdom is a good trade off. This is of particular note by the endgame when you'll pretty much be losing provinces no matter what you do, you need to focus on your goals and ignore the chaos in the rest of the country.
