Hi Janghwa,
You are smart enough to do your job.
You are not useless.
You are a good and valuable person.
You are not a bad employee, even when a mistake has been made.
These are all statements countering the negative self-talk in your post. The way that we talk to ourselve is really important...for a few reasons.
First of all, thinking positively about one's self is empowering. It boosts our confindence, and helps us reach our goals.
Also, it help to fight the anxiety and depression...they both feed on it...so starve them out!
Saying only positive things, even in difficult situations will make a huge difference in how you feel.
For example..
"I made a mistake, but I am seeking to fix it and learn from this situation."
Saying something like this is much heathier and more productive than beating yourself up!
If you are truly feeling over your head at work, having an honest talk with your boss about what can be done may be a wise step.
Until then, head up! :) You are awesome!!
S.C.