Do any thing long enough with any modicum of skill you had to work for and you're going to run into Imposter Syndrome. What is Imposter Syndrome (simply I.S. from now on in this article)? I.S. is the feeling that you are an imposter. It is the crippling doubt that you know you're a fraud and it is just a matter of time before everyone else figures it out. It is caused by, among other things, self-awareness of where you are weak combined with a fixation on those weaknesses to give them more credence than they should have.
I don't know a person who GMs good games that doesn't feel I.S. from time to time. Today I want to talk about that, and how you can try and fight the feelings.
Trust Your Players
This is perhaps the most logical solution, and also the one least likely to work when I.S. rears its ugly head and strikes at your heart. However, you should trust your players. If your players are having fun in your game, if they are being entertained, and if they are showing up every session eager to play more than odds areyou are doing a fine job. Your game doesn't have to be good for everyone, it just has to be fun for the group who is playing it. As such, your players - and you - are the only judges of quality that matters. Embrace that. Take solace in it. Wield it like a club in the dark recesses of your heart to beat I.S. back into its cage.
Just Do Your Best
Odds are you aren't GMing as a professional, and that brings with it certain freedoms. Among those freedoms is the fact that no one is going to certify you for being a GM, nor is anyone going to take your books, dice, and GM screen away if you aren't the most amazing GM ever. All you need t do is your best. There isn't a "not good enough" when it comes to GMing, so don't worry about that.
Remember: Bad Games Happen
A bad game is not a reason to beat yourself up. Everyone has bad games. They are a fact of life. Not every game can be better than the one before it, or even as good as the one before it. At some point you will run a game that is disappointing, just like at some point you will run a game that exceeds every expectation you have. Know that bad games happen. Forgive yourself when you have a bad game. Don't carry the weight of it forward, just learn and try to do better next time.
There are other ways to fight I.S. but these are the three most common for me. Ultimately, GMing is about having fun, telling a story, and arbitrating rules. There is no wrong way to do that, just ways that work and don't work for your group. Don't let a bad session ruin your fun. Don't hyper focus on your failings or weak areas, as tempting as that may be. Do your best, and try to have fun. Odds are if you are having fun, your players are too. Embrace it, use it. You got this.