I worked in a international research institute and would consider myself fluent but I still don't dare to say squirrel.

In general I struggle with any word that has the Z sound on it, since in spanish we don't have it, like Because. but also certain professions like otorinolaringologist! optometrist, etc etc

I am a native speaker, but for some reason I have a devil of a problem pronouncing the difference between "feel" and "fuel."

It's difficult for me to hear the difference between "flour" and "flower".Also in Japanese,"R" and "L" are same pronunciation.For example,work and walk.

First language has a great influence on second and third language. People have difficult in the pronunciation of the sounds which are not present in their first language. In my case, there is no f sound in Pashto language. Therefore, I often pronounce f as p, but the good thing about me is that I know how to pronounce it.