Matching megamindbrian2's comment, it never really goes away in my experience. However, some coping mechanisms I use are to read about new things in my field, p

Matching megamindbrian2's comment, it never really goes away in my experience. However, some coping mechanisms I use are to read about new things in my field, play CTFs (even if I do poorly, just for fun to learn from write ups), and practice my reinforcing what I do know. I find that doing things like that reinforce my confidence and give me the drive to go learn new things and build my knowledge. Also try to surround yourself with good people, that's easier said than done, but even following colleagues on social media that you find to be good role models is helpful. It's reassuring to me when someone I really look up to also experiences these same feelings. It's just comforting to know you're not alone sometimes in that regard. But as far as shaking off the "nagging feeling" goes, I'd start with reinforcing what you do know to help build confidence, and see if that helps.