It can be quite common after receiving a belt promotion to feel that you don’t deserve your current belt.
Perhaps there are some techniques you can’t do or are bad at or perhaps there are lower belts in your club
Belts are analog not binary
Sometimes this feeling can come about because of the rank of other judoka at your club. If there’s a judoka with the belt you are being promoted to that you think is particularly skilled. Because you are now the same belt as this person but they are very good it can cause feelings of inadequacy. It’s important to realise however that skill within a belt is a continuum and that different people will be different distances along this continuum.
It’s not about who you can beat
Sometimes struggling against earlier ranks can trigger doubt. However the belt is not necessarily an indication of who can beat who. Everybody has different strengths and weaknesses. Sometimes the style of someone’s judo can cause your style issues. Furthermore no two judoka are the same strength, size, weight e.t.c. Being beaten by earlier ranks is not (necessarily) a sign that you should be a lower rank.
Dunning meet Kruger
Doubting your ability may, oddly, back up the fact that you are skilled. The Dunning Kruger effect is a pattern of thinking where unskilled individuals assume they are better than they are because they don’t know enough to recognise that they are unskilled. The corollary to this is that skilled individuals often underestimate their skill. This resonates with me. I feel like the more I learn about Judo the more I realise that there is lots that I don’t know.
Coach knows best
Assuming that your club and coach is legit then you wouldn’t have been promoted without good reason. Most countries usually have syllabus and and criteria for you to be deemed a given belt. Since you are (probably) not as experienced as your coach you can defer to their expertise.
Work for it
Feeling insecure about your belt can provide a great motivator. Striving to grow into your belt can provide a great catalyst for you to really work on and improve your judo.
In the end a belt is best for holding up your gi. If you are improving and enjoying judo then try not to stress too much about a piece of fabric.