You are not stupid. These things happen, and sometimes when you start thinking about it, it overtakes you. That doesn't make it okay, but it happened and you now know that this isn't something you want to do again. You are still a strong, lovely person. Try not to beat yourself up for it and instead learn how to improve and prevent this from happening next time. I struggle with the thoughts all the time, and I'm sure if I was in the same situation as you with your coworker, I may have even slipped up myself.

You made a mistake, and you are still an amazing person, never forget it.