I purchased Heir to the Throne some time ago, but never really played it. Recently I've been playing though, and my current game is as the Teutonic Order, and for some reason I can't beat anybody in any war. No matter if I outnumber them or not, I always lose, and they instantly move into the same province that I'm retreating to so they completely erase my troops after one engagement and I can't see how anybody can possibly win like this. I managed to annex Pomerania, Riga, and scored a few territories in a land grab against Lithuania with several other countries distracting them. My force limit is 18 regiments, and I'm maxed out on that, and 22 ships, which I'm also maxed out in. But it doesn't matter, they somehow can sustain 40,000 troops and more, and they instantly defeat me anywhere. I fought Sweden, with 2 carracks and 14 galleys against their 12 galleys and they rapidly sank my entire fleet. I don't even understand.
I never played Httt, but I played both Complete and chronicles and your problem doesn't seem to concern the difference between those version so I try and answer.
There are several things that affect the outcome of a battle onf of the things you figured out:
1. Numbers. The more men, the greater dmg you inflict on your enemy and you increase your chance of winning.
2. Land tech(or sea tech for naval battles). Better tech gives you more morale and more dmg both at shock and later also on fire. It also increases your supply limit. Some techs gives a bigger advantages than others. Land tech 18 is the first one where one difference in land tech will matters a lot.
3. Generals. Generals can be purchased for the cost of 1 diplomat, around 50 ducats and a loss in land tradition. The higher the land tradition, the better the general. In the early years, fire sucks as you do next to no dmg, so shock generals is what you want. Manouver helps your troops moves faster, wich is usefull for defending bonus(see next point), and it also helps against attrition. Every point in manouver reduces attrtion with 1%
4. If someone attacks you, there is a chance you will get a defender bonus. Crossing a river gives a bonus at 1, defending in a mounting gives a bonus of 5 I think, forests gives a bonus of 2, plains do not give a bonus. If you hover your mouse over a province in the terrain mode you can see the chance of getting a defender bonus in that area. If it says 66% plains and 34% mountains, you will in 1/3 of the times get a defender bonus of 5(or more correct the attacker gets a -5 malus), the other 2/3 of the time you will not get a bonus. If it showes a river you will sometimes get the river bonus and sometimes not. So if you're going to loose a battle and you have a leader with a higher manouver bonus than the enemy, retreat them in to a province with mountains if possible so you atleast get a good chance of getting a defender bonus.
5. Upgrading units. Some units are better than others. As you go up in tech you will at some tech lvls get new units to choose from. Some will be good at defending either at fire, shock or morale wich means they will loose less men and morale then other units with fewer pips. If they got many pips on offensce they will do more dmg. One thing to make note of though: DO NOT change units in the middle of a war unless you're 100% sure you won't fight any battles for atleast 2-3 months becouse it makes your tropps loose all their morale and be an easy prey for enemy troops.
6. Morale. Several things affects morale, one thing is at mentioned earlier is land tech, the pips on morale helps, you can have the national ide that gives + 1 in morale, some religion gives + in morale as do some decisions, if you go towards land on your land/naval slider you will also get some bonus there. Ofcourse it's always the maintenance slider. When at peace it
should be at 50%(the lowest possible) to save money, but at war you need to put it up tp 100% or you will most likely loose all the battles unless you are vastly superior in tech lvls.
7. Dicipline. Troops with better diciplinces fight betters than troops withouth.
8 . And finally the thing we all have choose to love and hate, and that I know have caused many EU3 players to quit and realod without saving: LUCK. If you constantly rolls 0,1 and 2 and your opponent rolls 7, 8 and 9 you need to have a pretty good lead on him otherwise to win that battle. A general with 4 points in shock will add 4 points of roll value in every shock face. If the enemy also have a general with shock, but he only have 2 points. You will get 2 points bonus and he will get none. The roll numbers is also the same as the malus you can get from the terrains as explained in 4.
So as you can see, numbers is just one of many things that affects the outcome of a battle. If you go through that list I hope one or several points helps explains to you why you loose your battles. On extra trick that doesn't effect a single battle, but can better your odds in front of it is attrition. Every province have a supply limit. If you controll ans enemy province that numbers gets muliply with 3, same goes if you got MA. If you OWN and control a province you get to multiplicate with 5. Each whole numbers equals to 1000 troops or 10 cannons. For every 1000 men you go above the support limit,your troops will loose 1 ekctra % at the end of the month. Attrition doesn't only reduce the number of troops you have it also increase your WE, wich among other things increase your revolt risk, wich will ofcourse cause more revolts, but it will also lowers your tax income.
So how to use this to your advantage. Prawnstar got some nice tips in his AARs on attrition, and while I don't reccomend trying to play the game like he does, some of the tips and tricks he uses can be usefull in somewhat easier missions. One thing is to fight at home grounds. Let the enemy come to you, and let them loose men while siegeing in your provinces, if possible, scorch the earth and retreat so the attrition will be bigger. If they assault and fail, attack them shortly after, while thei're low on morale and are easy pickings. Go after then and retake the forts when they only have 100-300 defenders in them. If you get the chance attacks small stacks. This will both improve your warscore and take out some of the tropps without much loss of your own. When you think their doomstacks is beeing reduced enough for you to take them on, go in for the kill, with your mains tack and best general. When most of their army is gone, then it's time to move into enemy territory and start to carpet siege.
Also watch your own attrition, don't go around with doomstacks of your own in low supply enemy provinces, don't assault, if you don't have atleast 10 000 men for every fort lvl, as you will loose a lot of men. Avoid the enemy doomstacks until they are reduced from attrition. Rest in your own provinces. Ther you will get 200 soldiers back pr unit at the end of the month. If you got MA or the enemy province is under your control, you get 100 soliders pr unit, in an enemy province you don't control, you will only get 20 soldiers pr month, pr unit. This will not help your WE, but it will help you get back to full strenght faster, assuming you got the manpower for it.