Not claiming to be Henry or Skimbo... But...
1. Pick max 3 defensive shells (1 thats good against the pass, one good against the run and one thats flexible.
2. Learn 3 plays in each shell (one in man, one in zone and one blitz)
3. Whatever happens stick your shells and plays for 20-odd games and have a reason for calling each shell and play (down, distance, opponent's formation, time of game, etc)...
There are games where even the best players get dogwalked and other games where even noobs have a field day. I discovered that constantly changing my scheme/approach from drive to drive and game to game was counter-productive
You will get better when you do the same things over and over for a while (getting better at it)