We usually feel that when our potential is not satisfied and usually I think people who have worked on their own end to end projects in companies or have done start ups have a sharper antennae to know when they are wasting their time.
But it could also be that you are wearing the entrepreneur hat while being an employee, that can cause immense frustration. As an employee just be a great team player, do the work assigned and most of the times try to be in 100-110% range of your output and go home. Then work on your thing, where you wear the entrepreneur hat. This is a nice trick that works if you push yourself a bit; detach from the expectation of fulfillment from a job at least for now.
If nothing works, then you got to find a new job but before that you would seriously think on the type of problems you would enjoy working on, verify that with side projects or know that from your startup experience, apply till you hit the right job. And if job is not a cup of tea, then part time remote consulting and working on your own thing.
The point is you need to chill out and as others have pointed out, first be happy that you are in a job at one of the top 4, enjoy the 8 hours your spend there and use it as a leverage to course correct instead of throwing the baby out with the bath tub, though I know you won't do that. Good luck!