You're doing well whenever you can say to yourself that you put in a fair day's work. To ask yourself if I'm content with what I accomplished today, and then come back with a confident answer of "yes", is to say that today I can stand proudly behind my work: I did what I promised myself and others. I didn't waste mine or other people's time. I stayed on top of everything important.
You are probably not able to define a fair day's work but I'm pretty sure you know it when you see it. Fair is sustainable. Fair is calm. Fair is not stretch. Fair is not perfect.
You can have people find out that you're not perfect and still put in a fair day's work. But it's not easy to set your mind at rest if today you let someone down, if you got caught in busywork, or if you're constantly afraid that something is going to fall through the cracks.
It's going to be much easier to avoid these things from happening if you stop overpromising, if you allow yourself to fail, if you don't work long hours for the sake of appearances, and if you learn how to stop thinking about work during your non-work time.
"Did I put in a fair day's work" might be the best question you can ask yourself today before leaving the office.