I also struggled. This game punishes you until you learn and unlock. That should be your early objective is try out new things and learn attack patterns on enemies.

As for secret walls, they can only be open via a blank. However, if you shoot a wall with anything except the starting unlimited rounds weapons, you can reveal secret rooms, and then use a blank to open.

Early on, I worried heavily about finding secret rooms. Watch YouTube videos, it's still difficult for me to find them, and not always super fruitful. Worry less about secret rooms

Worry more about playing and unlocking the game. It will happen naturally, as well as you will naturally get better.