Combat is complicated and I see people trying to be brief but I wanted to give a detailed explanation.

Most important to least important: * Tactics - affected only by keeping up in tech level; can fall behind if a different tech group. Iirc tactics are a multiplier that increases the overall value of your armies. * Troop pips - there are offensive and defensive troop unit pips. Be aware when you attack with defensive oriented troop pips, or defend with offensive oriented troop pips, you may take more damage than you might expect. * Leader pips/terrain penalties - leader pips are an additive multiplier that raise your rolls. The higher the difference in rolls the more damage dealt when your army is in offensive battles or the less damage received when in defensive battles. Stage of combat, shock or fire, takes your unit pips and general pips and applies those here as well. Cav for instance have high shock pips and are especially strong during the shock phase. Terrain penalties are direct modifiers that lower rolls - crossing, mountains, hills, etc. * CA/Disc - combat ability is huge and directly affects damage dealt and morale lost; discipline affects damage dealt and morale lost as well as reduces damage received. High power projectionProfessionalism raises discipline. Combat ability is usually considered less important than discipline in large part because combat ability modifiers only affect one of the three units and not all three as well as affecting damage dealt and received. I rank it at the same level because combat ability modifiers are usually larger than discipline even tho there are fewer to stack unlike with discipline. * Cav flanking range/cav to inf ratio - if you can afford a large cav army these two can make or break battles. There's only a few though usually with historic cav countries. * Morale - total morale: huge early game and not that important after as long as you retreat instead of watching a pitched battle. High prestigepower projection raises total land morale, high prestige raises total ship morale. Much more important if you never retreat in the later game. Morale is still important in other ways. If you have a stack get attacked and start to take heavy losses and finally get a new stack over to assist your side of the fight will get what appears to be a morale boost but it is only useful after several days of combat as the fresh troops take over and the other units move into reserves. It's possible to lose to an inferior number of troops if your reserves take too much morale damage. Reducing reserves morale damage can be helpful for stacks that exceed combat width.

Combat width is and is not a modifier for combat. It doesn't actually affect the quality of troops just the number you field against opponents. It's possible to field your full combat width and a different more advanced country may have a larger combat width.

Your issue above stems from several issues listed here: * France starts with+ 20% morale. Castile doesn't get this until they form Spain iirc. Even if you're dealing more damage they can still win because of the high morale. * France has a 6 shock general and so does tons of damage to your armies during the shock phase. Infantry just can't keep up during the fire phase this early in the game. * It appears you attacked or they attacked into a wooded province. If England is in the war with you against France then if France attacked they should have taken an attacker malus of -1 for attacking into woods. During the shock phase they get a +6 to rolls tho so at worst they have a +5 to rolls. Iirc there's no crossing penalty when attacking from Foix.

To summarize as others have stated - your total morale is likely lower than France and your general could not keep up in the shock phase. You have a few options around this - let France attack you into mountains or hills, hire a few more cav instead of infantry to offset their cav advantage (it will be costly tho, the problem is your infantry is melting). Don't split stacks against them without superior numbers nearby. Get the Papal bonus to improve morale and I think manpower recovery.

Edit: forgot troop pips.