One thing you have to be prepared for as a GM is that not every session will be perfect. Not every session will be great. Not every session will be as good as the one before it, or build on what came before. Not every session will be good, and that's ok.
A session can have any number of problems. The problems could be around you as the GM - maybe you're burned out, maybe you're feeling uninspired, maybe you're too busy with other things to prepare, maybe you just prepared wrong, or maybe none of these, or all of these. It could also come for other reasons. A key player suddenly can't make it. The players are all in a mood where they'd rather go off topic and joke then play - and maybe this is even worse because you're trying to have a serious session and they're cracking wise.
It doesn't matter. And it happens in everything you do. Some games are just better than others. Some games are just worse than others. Ultimately, at the end of the year, one session has to be the worst session of the year. And maybe it is horrible, or maybe it is not.
But here's the thing.
You can't let it get to you. What you think is a horrible session now, could turn out to be the key to the best session ever. What you think is a horrible session could be someone else's favorite. Heck, it could even be a great session when looked at later.
Some sessions will be good. Some sessions will be bad. Some sessions will seem good only to feel like maybe not so much later on. Other sessions will do the opposite.
There's nothing you can do about a session once it happened, it's in the past. So just take what you can and go forward. Try to make the next session as good as you can make it. Don't worry about being better or worse. Just try to make the game fun. It's all you can do, and in the end all that matters is that overall people had fun with your game.
You can do it.
I can do it too.