Circuit breaker for single outlet
This is complicated since the downstream breaker must be faster then the upstream breakers. It needs to have different time characteristics.

This is called breaker selectivity.

Without replacing upstream breakers, you'd probably need a breaker that is so sensitive that you can't even enable the breaker. The inrush would trip it immediately.

Also, a 20A breaker won't trip immediately at 21A. This will take minutes. See the image in this post.
I think you have a larger overload.