Why am I having these negative thoughts about myself?

Because your inner critic is being overactive, and is not saying, that didn't go well, it is saying 'that was terrible why did you think you were good enough for that

Because some stupid chemicals in your brain are malfunctioning. Remember: This is not who you are! Having negative thoughts and doubts is normal, but if they become too much it is more like a sickness.

Negative thought and self-doubt is very common thing that every person experiences from the poorest to the wealthiest. Its completely ok to have 2-5 low days in a month the important thing is to bounce back harder and try improving your skills for your longterm goal. Having your thoughts written in a diary or could be even on your smart devices in its notes section (phone, laptop, tablet). This helps you to clear out your mind for that time and plus you can always get back to it and try to understand why did you feel in a such a way and maybe work on it.