Reset chess knowledge

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Smoggyabbainopadano

Sometimes i changed my opening repertoire in past,and studying a new opening is like to reset my chess knowledge.Now why after 5-6 years that i play i lost 4-5 years to change opening?Do not make my mistake,study middlegame and endings,that knowledge will always be helpful to you.It has no sense to memorize 30 najdorf theorical moves with black and then change in carokann for changing later again etc.No waste time to study opening,but do tactics,strategies,middlegame and endings.Learn from my mistakes,no waste like me 4-5 years of your life!Thx.