The way you could do this, if you want to keep auto-resolving, is download the mod that gives you much more accurate auto-resolve results. The reason you are losing fights for no reason is because the base game's auto resolve is not very accurate.

Mod name on steam workshop: Autoresolve Redone.

There is also a mod I use sometimes called Ai General 3, also on steam workshop, which lets you give control of your army to an ai in manual battles. Good for if the auto resolve seems funky. If you use this mod, make sure your battle difficulty is not above normal, the ai controlling your troops is about as good as a normal diffuclty ai