Despite our best intentions, sometimes a session just doesn't go how we intended. Maybe your players didn't latch onto the NPC you thought they'd love, or they ignored a story hook you'd been planning for a while. Maybe the session was a bit chaotic, or awkward, or never really sparked into life. I get it. This happens sometimes, and it sucks.
You really put yourself on the line by DMing, so days when it falls a bit flat, it's easy to take it personally. As if you're not doing a good enough job and the effort you're making is not being appreciated. If I can offer you any consolation, it's that we've all been there. It's not always easy to bounce back from a disappointing session - but here's a few things to keep in mind which should hopefully help a little.
Take a break
First things first, don't spend too long dwelling on a bad session. Give yourself a bit of mental space: watch a film, go for a bike-ride - whatever takes your mind off it. I know I have the tendency to beat myself up if a session doesn't go as well as I like and I can spend a long time feeling frustrated by it - yet when I'm in this mindset, I am liable to catastrophise everything about the session, and am definitely not in the headspace to try to learn from it.
To try to combat this tendency, therefore, I give myself license to not think about it for a while. Instead, give yourself a bit of headspace and come back to it a day or so later. You'll probably find that you were over-reacting a little, and now are in a better frame of mind to try to judge it rationally, and to see where you can improve in the future.
Get some perspective
As many things in life, regardless of how badly you thought it went, it was probably nowhere near as bad in reality. We are so often our worst critics, and being a DM means you've got so much going through your head at any point, that you're probably the worst person to try and judge how your session was perceived by your players.
If you have a good relationship with your players then, you could consider asking them for their thoughts and feedback on the session. And usually when I do this, my players' perspective is far more positive than my own. Of course, this could just be down to them being nice and not wanting to hurt my feelings (which is why it's important to make clear that you do value honest feedback) - but for the most part I think it's just that the experience of playing and DMing is so different, that sometiems it's tricky as DM to accurately assess what it's like on the other side of the screen.
It's not all on you
It's also important to remember that while you as DM do shoulder a lot of responsibility for making the session fun and enjoyable, you don't shoulder all of that responsibility. Your players need to meet you halfway - and if you're the only one making an effort in the social encounters, or your players are just checking their phones throughout, then any lack of energy during the session is at least partly their responsibility as well.
This isn't an excuse to get frustrated with your players or demand more of them than they can give (everyone has their own lives, struggles, and energy levels - and maybe they just didn't have the energy for that particular session) but it does mean that you can cut yourself a bit of slack - as any disappointing game nights aren't just down to you.
It can't always be great
And even if it was solely down to your DMing, and your players agree that there was room for improvement with that session - that's fine too! Listen, not everything can be amazing all the time. Sometimes it's just good, or good enough. With any skill that you're learning, progression isn't a linear process - it's bumpy and their are days when you just kinda suck. That's ok - it's all a part of the process!
Not every session is going to be the best session you've ever run - and you have to be ok with that. And the very fact that you're disappointed when things aren't as great as they could be, means that you at least have the desire to improve them. This is a good thing! To do anything difficult in life, first you have to have the desire to achieve it. The fact that you're disappointed when you don't hit those high standards shows that you definitely have that desire. Which means you're ready for the final step: working at it.
Learn from your mistakes
If there really were shortcomings in your session (and, again, they probably weren't anywhere near as bad as you thought!) then this is the perfect opportunity to learn from them to improve your DMing. Were things a bit awkward when you had to make a ruling on something you weren't sure about and a Rules Lawyer started debating you on it? Great - this is an opportunity to go away and learn that rule. Was there a battle which felt a bit anti-climactic? Cool - you can think about how to improve your next combat session. Was there an NPC crucial to a bit of plot development that they didn't really engage with and talk to? Awesome - go and think about how to make your NPCs more egaging and dynamic.
Whatever you felt the shortcomings of your session were, that offers you a very tangible route to go away and improve a specific element of your DMing. Given that you're even on this site, that's probably something which is important to you, right? So don't feel down about a mediocre session - feel good that you're gonna discover a way to make your next session better!