I suppose the feeling itself came from comparing myself to others (which is natural), and it was having some success in some field relative to others that gave me the feeling like I could succeed, I wasn't behind everyone in everything, forever. I guess for the mental health aspect of this, it's probably good to pick something you are good at, better than others, and actively appreciate it. It's not good to feel completely behind in all things. Good luck! :)
To be honest I've felt like this since I was 18 and it never went away. What made you realize this wasn't the case?