how to do things even when you have immense anxiety?

everytime i'm learning something new of considerable importance (for example, a new concept for my professional career), i become so overwhelmed and have to stop myself from freaking out. often, it's because i'm finally facing what i don't know -- or even worse, i'm finding that i don't know what i don't know (as in there's significant gaps in my knowledge that i wouldn't even know how to start covering).

because of these feelings, i often procrastinate which makes me feel even worse. i'm really trying to become comfortable with not knowing how to do things, and learn how to take things head on. could anyone relate and perhaps provide some advice for how they got through this? i understand in life i will continuously have to confront things i don't know -- i want to remain level-headed.