TimDogg,
I sympathise! I found it workable to bulldoze as the Brits, using big stacks who got better with experience and refitting etc but long term it was hard to match the varied enemies you're bound to come across in a campaign game.
Fighting bandits helped boost Generals, hiring mercenaries-particularly effective cav or stable infantry works but numbers are limited in a given area.
But most of all I tried to keep experienced units together by refitting.

One great thing about the game is that skirmishing works.
On the defensive I used them as a screen to break up or delay what I hoped was main attack, and pursue routers.
On attack they're flexible enough to protect you on way forwards, hem the enemy in or at worst get sacrificed while a decision is being forced somewhere else..

Like you I found a disciplined/form a line type approach hard as most of my lot were raging for battle and dashed into a fight-It seems important to point your guys at something from the start, seems to stop them dashing off after distractions. But it means you're constantly keeping an eye on all units. And always on the move.

But smash the Dacians! The Eastern steppes are a great source of manpower and good mercenaries which work well against Rome and Greece who as has been pointed out lack native cav..

Good luck!