Decision fatigue

I am working on staying on my food protocol. The good thing is that I have become much more aware of what I am thinking. But in situations when my brain goes back to the old thoughts – we can get away with this one, one last time etc, I jump in and try to immediately replace the old thought with an intentional one. But to do that it creates an internal dialogue. For example,
Monkey brain – I want this chocolate;
Pre-frontal – hey you are not in charge; we have a protocol and I know why I want to stay on the protocol;
MB – but just this one, it won’t matter
Pre-frontal – we don’t do that anymore, you are not in charge

This dialogue helps to not go down the monkey brain road, but at the same time, I’ve noticed that it require so much energy of me that by the evening I am already exhausted and just don’t have the energy left to take the decision not to listen to the monkey brain. Taking active decisions throughout the day feels very exhausting.

Am I doing something wrong? How do you learn to think new thought without going into decision fatigue?