Why is it so hard to come out?

I am gay, but I am having such a hard time coming out to my friends and family. I know that I will be loved and supported through everything, but I cannot get the words out. My mom has always told me that I could tell her anything. I recently got out of heterosexual relationship, which is definitely not for me. I dream of a day I can just tell my friends and family. Any tips for preparing me?