Waaaay back when I was in AIESEC (fervent sort of student sales-training / exchange organization) someone drew a fantastically simple little diagram. It was three circles, one inside the other.
The inner-most was labeled "comfort zone." The second was labeled "discomfort zone." The third was "panic zone." Outside the third was "the universe."
The presenter argued that you never got rid of your "discomfort zone" or your "panic zone," but you could slowly expand your "comfort zone" by venturing into your "discomfort zone," essentially making things comfortable that used to be uncomfortable to you, and stretching your boundaries. As long as you don't stray into panic zone, you'd be ok. And as long as you keep doing this, your little circles would encompass more and more of the universe.
So an example - you're trying to learn guitar. You are extremely uncomfortable about playing in front of other people, so first you force yourself to play in front of your romantic partner. That eventually becomes comfortable, and so you play in front of some close friends. Over time, you're finally comfortable enough to play an open mic night... and so on.
But, if you had gone straight from beginner to open mic night, with nothing in between, you might have gone into a full blown panic, and permanently associated guitar playing with the sheer panic scenario of open mic night with no prior context, and forever ruined the hobby for you.