I made the mistake

I always think that it's a piece of cake to me about my work. It's too simple to care it. However, I had made a serious mistake. I am always making careless mistakes these days. I couldn't forgive myself and I wonder what the monitor will say.

Why? why 's that?  I asked to myself again and again, but I still can't explain why.  I don't like to make an excuse for myself.  

I've got a lot on my mind. It's too badly....

What can I do?  What  can I say?  I can't do anything and don't want to do anything. I only keep silent.

I don't think I will do that again. This taught me a lesson.