You could think of pronouns like this;

Imagine that you have a friend you've known since you were little. You've always called him "Johnny". You're now both in your 20s, and he's asked you not to call him "Johnny", but to call him "John". You might slip up now and then and accidentally call him "Johnny", but so long as you correct yourself and apologise, you should be fine.

Don't ever be afraid to ask for clarification, this shows that you want to respect the person and call them by what they want to be known as.