Why am I not improving?

Your body changes as the result of your actions. Some actions bring positive change, like practicing yoga. Some actions bring negative change, like working in a poor posture.

How much you improve is a simple equation - the sum of your positive and negative actions. If you do some positive actions, but do more negative actions, you will not improve. If you do some positive actions, while not changing your negative actions, you will improve slightly. If you do more positive actions, and do less negative actions, you will improve greatly.
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Doing positive actions do not give you excuses to do negative actions. If you want to truly improve - both physically and mentally - you should progressively add positives and remove negatives.