So lots of people play deathmatch and say that they never improve, here is why.

Reason one is you have audio in DM on, if you are playing audio on with no music than you are really just practicing listening to enemies and not your reaction to seeing an enemy. Turn audio off or on low and play with music.

Reason two is you need to practice your peeking and not focus on just surviving. I am immortal 2 and in DM I always have around the same amount of deaths as kills as I practice my wide swinging. I never try to timing the enemy I just try to practice swinging and snapping to the enemy. Never hide or shift walk in DM.

Reason three is that you are crouching and spraying in DM. This does not teach you anything, this is most likely what you already do in comp. Go to settings, unbind crouch and than load up DM and do headshots only. Never go for body shots and focus on hitting the enemies head even if you are dying doing so. It is better to die practicing than to live not learning.

Just practice in DM and play better in it than you do in a normal game and through time it will leak over into matchmaking.