Thanks to everyone who has provided some advice on this.
After watching some let's play videos for other ageod games and even reading some of their manuals, I think I'm starting to get more of an idea as to what the important things to control are.
It does however also seem that sometimes you are facing an enemy with superior troops or a general with a strong attack bonus, which means you will lose despite a numerical advantage.
As a test environment, I've been using the Risorgimento scenario in PoN as the fighting seems to centre across a very narrow front.
The invading Italians were initially defeating my numerically superior force but I'm now doing much better.
I don't know if this accounts for the difference but I've moved the 3 star general out of that area entirely as he had a zero defensive score. Instead I've been using my 2 star general that has a higher defensive score.
I'm still taking heavier losses than the Italians but my stack seems to be holding the province this time even when the battle is showing as an Austrian defeat.
My general with zero defensive score has a decent attack score of 3, so instead of using him to defend I sent him behind the lines and destroyed their rail network just behind the front line