Creative Fatigue is when you get mentally tired having to generate new, creative stuff, in the moment during play. Some game systems this is more, or less of an issue.
Two things influence this:
1) Iterations vs. outcome
How many times do you have to come up with creative descriptions for a given situation? If a whole situation is resolved in one dice roll, you only have to do it once. If you have to do multiple attacks, over and over, then you have to do it over and over.
Consider- how many attacks does a combat typically take? Over how many players? And each one should sound a little different than the other ones? Over how many combats? Over how many sessions?
2) Fiction Support
Different systems give you different levels of support on the mechanical level. If it's just a generic "Attack roll" that results in "hit or miss" you don't have a lot of support.
If the player has to choose between specific attacks ("Upward swing", "Change and bowl them over", "Grapple for a headlock") you now have specific events already chosen you don't have to create in the moment. If the system gives you support in what the outcomes look like ("Solid hit, but you take a glancing hit as well", "Opponent falls down", "They drop their weapon from pain.") you don't have to imagine those bits either. These things become the skeleton around which narration sits, and if solid enough, it even means you may not have to imagine much around it at all and still have fun.
As you can see, the two issues above are pretty much dictated by the mechanics you use. Aside from going to a system that better supports you, your options either become "Let's stop trying to narrate and simply speed through the mechanics with 'I hit I miss'" or trying to off load some of the creative needs to the players - "Tell me what happens" but that, too, often still results in creative fatigue, just a little more spread out.
You could try less/shorter combats and sessions - I know for my group we typically do 2-3 hour sessions and get more fun because we're not reaching points of wearing thin and getting tired creatively.