If you're not familiar with the acronym FOMO that's totally fine (and also a tiny bit ironic). FOMO stands for "Fear Of Missing Out." You might feel this for example if you're deciding whether to learn ReasonML or keep going deep into JavaScript (this happens to be something that I'm currently struggling with).
What's wrong with FOMO
FOMO is paralyzing. I once entered a conversation with a man at a conference who was very concerned about which JavaScript framework to use for building applications. He asked me how I could possibly decide. I just said that I tried a few and chose one that I thought was pretty good and went with it.
He couldn't grasp it at all and showed me a Google Spreadsheet he'd created that would probably take half a tree to print (a small tree probably). It had columns for every framework imaginable and rows for every conceivable feature. I'm telling you, this thing was HUGE.
Even with this exhaustive comparison, he still couldn't decide. He was worried about making the wrong decision. We call this analysis paralysis and I actually mentioned to him that I thought he'd hit that point which he denied. Whether or not that really was the case, he was emotionally consumed by this decision.
Where does FOMO come from?
FOMO is entirely based on comparing one's self with others. These kinds of comparisons are entirely unhealthy and insatiable. No matter how much knowledge or experience you gain, there will always be someone who has more and you will be left feeling inadequate.
The reason for this is we often make the mistake of thinking there are only two people in the world: You, and everyone else. While this entirely false, it's an easy pit to fall into and leads to FOMO and feelings that we can never measure up to where everyone who is not us is at.
How to manage
Knowing that FOMO comes from the unhealthy tendency to compare ourselves to others, managing and gaining control of FOMO becomes a task of controlling that tendency. I'm still working on this myself, but I think a great first step is to be mindful of yourself and the thoughts you're having about others and yourself. Try to catch yourself thinking thoughts like: "I'm better than them at x" or "Wow, I'll never be that good at x as they are." However true those thoughts are, they are not helpful or compassionate to yourself or others.
Instead, consciously train your brain to think things like: "How can I help others learn what I know?" or "Wow! That's really cool that they're so talented at that!" There's nothing wrong with a healthy desire to learn more, and you can train your brain to think positively about your ability to improve. Changing the tone and attitude of your own self-talk can really make a positive impact of how you feel about yourself and others. You'll feel more motivated to improve and feel empowered to do so.
Conclusion
I think it's really important for you to know that your brain is a muscle and you can exercise different parts of it. Try to exercise the parts of your brain that help you be more compassionate to yourself and others and you'll reduce your unhealthy FOMO and develop a healthy amount of MTBB (Motivation To Become Better... I just made that up).
I hope that's helpful to you. Like I said, this is something I'm actively working on in myself, I hope that together we can work on improving this aspect of ourselves and find more happiness in life :) Good luck!