Can't figure out what I'm doing wrong

[Actuarian]

It sounds like your problem is almost entirely due to you micromanaging the tax levels of provinces. In general, you shouldn't take control of the tax level of a province unless you have a specific reason (blood hunting province, pillaging, are there any others?). It does a good just of maximizing revenue while controlling unrest.

There is absolutely no reason to turn off taxing for provinces with low incomes. You're just hurting yourself. Let the computer manage the tax level and you will get something out of those provinces.

To the extent that your problem is resources, consider buying troops from conquered provinces to fill up your ranks. Heavy infantry, light infanty with javelins and archers are all pretty useful. Even if you can only buy one or two of these per province per turn they can be combined with your existing troops to make a big difference.

Summons can also help if you need more troops.