No doubt on your karate journey thus far you have experienced times when you are not improving. To have achieved your Blue belt, it’s undeniable that you would have experienced this feeling at some point (perhaps multiple times). And unlike those who quit, you worked through it and overcame. For that, congratulations!
But now here you are again and perhaps this time the feeling is stronger than it has been in the past.
One reason why this feeling strengthens at Blue belt is because there is a greater time period between gradings. In the past you may have used a grading as a short-term carrot to keep you persisting. Equally, in the past your kata only had 20 counts to get develop, now you have a kata with 38 counts.