When I was in Moscow, Russia as a business trip, I and my colleagues used to take a taxi for commuting. Since the business trip lasted over 10 months, so we were really familiar with doing it. When we took a taxi, we would give the destination and desired fare to get there. If the driver accepted the condition, the negotiation was completed. Even though the drivers could not speak English, there was no problem because I could write down the destination and desired fare on the paper, and most of the drivers understood the meaning. One day, we took a taxi in the routine way. When we got there, I was trying to pay the fare, however the driver denied the amount of the fare. He wanted 10 times more money than we negotiated at the beginning. I was really embarassed but I couldn't do anything. So I decided to pay the required money. I went the bank's ATM machine and withdrew the money and also exchnaged dollars to russian currency. And I was able to pay the driver. That was a terrible experience.

"I couldn't help myself" is usually when you want something and you feel like you need it.
--> Whenever I go window shopping, I always end up buying something. I just can't help myself.

This is also why we say "help yourself' in English. It means "take what you want"
--> These grapes are delicious!
--> Please, help yourself. (= eat all the grapes you want)