And that’s OK.
If you want to be good at anything, you have to suck at it first. This include game mastering. The unfortunate situation with game mastering is that you have to suck in front of your friends and they have sacrificed some part of their precious time to experience the suck.
Therefore, there is a lot of social pressure on you to get it right, the first time. It’s not going to happen though. You are going to fuck it up. You are going to bring a shit adventure to the table, especially if you are designing your own adventures.
Understand that and be humble. Even veterans have bad sessions. I know I do. I am always trying to get better.
It takes the sting out of it when you know that you are human, and everyone who is a game master has bad sessions. Every one.