I went through similar struggles when I first got the game way back when.
Your casualties are too high. You should never lose 20,000 men in one conflict. One thing that is pretty basic and fundamental that took me a while to take seriously and implement in my tactical philosophy is cover, and flank. Once you find cover, stay put and flank. Advance to cover, hold and flank, advance. This is simple, yet crucial to keeping casualties down.
Also, protect your flanks with skirmishers, cav, and light brigades.
Hope this helps. Good luck!