Personally for me, I know I'm doing everything I can when my superiors compliment and praise me for completing certain assignments. I also know I'm doing everything when I have a pile of work at the end of the day that I have completed. :)
Ask whoever is in charge at the time what you can do next, when you're finished a task. Before you leave, ask if there's anything else that can be done. Remember the responsibilities that were given to you for your specific position and go through them- are you completing them all to the best of your abilities? If you aren't sure how to do a task, or if you aren't sure what to be doing, ask questions!
Your feelings of adequacy are usually through our thoughts which frame our perception. This means that what others tell us in turn generate thoughts and then the feelings in response to that. The truth is, no can ever do every single thing right every time. Even the superstar CEOs have their days of inadequacies. The key is to focus on learning and be open to taking in feedback, constructive criticism. Your workplace performance is a part of you, it is not a reflection of the whole person you are. Keep doing your best and be open to feedback. However remember that you in yourself are enough. No one can make you feel inferior in any way unless you allow them to.
Just be sure to do the work you are assigned to do will ensure that you are doing what you need to do in the workplace. Also, if you decide to do a little extra, it can help you to feel like you accomplished even more above your call of duty.
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