Either try and do a challenge run on normal mode (No items for example) and it'll be closer to the difficulty of dark mirror but without losing your enemy knowledge. Option B is just keep at it, the enemies are different and the game is harder.
Secret option C (How mysterious!) Just don't play Dark mirror. If you don't have fun with it, then just forget about it until you want to try it again.

for me, try use a tanky build like shield skull or ghoul, or someone else with revenge or fortress inscription, and take a few run to see what they do, and get used to their rythm to dodge and such, bringing a tanky build to scout how things work has been helping me up to dark 6