You'll either realize it yourself or someone will let you know, but at some point, you'll find out you're not good enough.
You can no longer keep up, the field is changing, or maybe someone on the scene is better. Perhaps your passion has waned or your interests have changed. Your work isn't up to your own standards anymore.
This is a fine spot to be in, to recognize it's just not working.
Once we recognize that good isn't enough, we can move on.
We can move on to something better.
Good has to end before better can begin. So either improve until you're better or choose a discipline, a job, or a life where you can excel.
Good isn't enough.