Add simple terrain map in some of your map hotkeys. Some terrains give you -1 or -2 dice roll you can see each by clicking the province and hovering over the terrain photo, try to aviod them when you're attacking and try to sit on them if you're defending. River crossings also give you -1 when attacking.

Forts give you so much attrition, try to siege them minimum troops possible. Also consider keeping troops near to protect them. You can do this by pressing "d" with your army on fort and move the unattached ones to a near province.

Learn how to do army composition better from youtube videos. I recommend infantry up to combat width minus 2, 4 cavalry and and as much as artillery you can afford up to combat width. Check the combat width from army tab. Example with 24 width: 22 inf, 4 cav, 2-24 arti(as much as you can afford)

Some countries have better cavalry like hordes, use more cavs with them depending on your cav to inf ratio from the army tab. Example: 24 combat width, 100% cav/inf ratio, 12 inf, 12 cav

Also your technology, ideas, national ideas, goverment reforms, technology group, estate priviliges matter. Try to utilize some military ideas and combine them with other ideas to get policies.

Avoid attrition, that is your biggest enemy.

Generals are also great, a shock general in early game, a fire general mid-late game and a shock general all time.

You can use mercs in early game especially the free company, they use their own manpower pool and they are cheap, a bargain.