When you're driving, you shift into a higher gear so you can continue accelerating. If you continue accelerating at the current gear, you'll wear out your engine and burn up a lot of gas.
The same thing applies to people. You're pushing yourself, you're trying to do more and get everything done, and you can physically feel yourself reaching the limit of what you can do.
This is a sign that you need to shift to a higher gear, or you'll burn out.
How To Level Up
The first thing to do is take a break. Give yourself a chance to recover a bit. Your mind works similarly to your muscles, where if you push it past what it's comfortable with then give it time to recover, it will grow so the next you can handle a higher load.
Next, identify what kind of work you have been doing lately? Has it been primarily internal work like reading books or taking a class, or external work, like building something or shipping a deliverable?
The goal is to switch between these states regularly. Learn something new, then commit to doing something with what you've learned. Then go back, reflect on what you've learned and done. How can you improve, and what skills will help you be more effective? The cycle continues.
It's similar to how when you work out, and you alternate between upper and lower body days. Mix in a few recovery days, and you'll continue to level up.