‘Resting on your laurels’
When someone is described as resting on their laurels, it paints a picture of someone who has become complacent as a result of being overly satisfied with their achievements.
In ancient Greece, laurel wreaths were a symbol of status and victory, and one would only be wearing one after having accomplished quite a bit. Once you start resting on your laurels, however, you’ve become a bit lazy and perhaps too sure of yourself to apply yourself further.