Hindsight, as they say, is 20/20. We all do things that, later on, we can see didn't make sense. That doesn't make one stupid. And, although I don't know you, I doubt you're stupid. It does sound, though, like you perhaps have some self-esteem issues. Is it possible you might have ADHD or have you been diagnosed with it? Inattentiveness is, as I understand it, a symptom of ADHD.

I'm sorry you feel so down on yourself. Perhaps if you're seeing a mental health professional of some sort (a psychologist or mental health therapist) this might be something to delve into with them? Under any circumstances, I hope you will be able to find a way to not feel you are stupid & need to apologize for yourself. Best wishes...