Sudden regression after study videos


It's normal for learning new things to disrupt your play causing you to be temporarily worse. With some practice, you'll incorporate the new things into your play and be better.
Having said that, watching videos may not be best. Do whatever works for you. Solving tactics is good, but at some point you'll also need to learn about strategy, endgames, and openings.

It happens to me all the time when I want to "copy"/borrow ideas of GM games. But then, my opponents response with unexpected moves so I don't know what to do next, and blunder. That is because I did not fully understand the whole strategy. Focusing on one single idea, but not the plan, will often results in blunders.