Dealing with impostor syndrome isn't fun, but playing Dungeons & Dragons is! Here's a quick little 5e spell combining the two:
Impostor syndrome
5th-level enchantment
Casting Time: 1 action
Range: 60 feet
Components: V, S
Duration: Concentration, up to 1 minute
You create an overwhelming sense of doubt in the mind of a creature that you can see within range. The target must make a Wisdom saving throw. On a success, the spell ends. On a failed save, the target firmly believes it is less talented than it actually is, and its Intelligence score is reduced by 2 until this spell ends. Additionally, the target becomes very anxious, and any Constitution saving throws made to maintain concentration are made with disadvantage until this spell ends.
While a target is affected by this spell, the target rationalizes all previous accomplishments as luck and deeply fears its magic will fail and reveal it as a fraud.
On each of your turns for the duration, you can use your action to deal 4d8 psychic damage to the target. You do not need to be able to see the target or continue to be within range to deal this damage.
At the end of each of the affected target's turns, it can make a Wisdom saving throw. If you can no longer see the target, it has advantage on this saving throw. On a successful save, this spell ends.
At Higher Levels. When you cast this spell using a 6th-level spell slot, the target's Intelligence score is reduced by 3 on a failed save. When you use a spell slot of 7th level or higher, its Intelligence score is reduced by 4 on a failed save.
Available to classes: Bard, Sorcerer, Warlock, Wizard