Why do I feel like I just made a huge mistake?

Today, on my way to the job I hate, I made a stop at the job I love to drop off my letter of resignation. For once in my life I was trying to think logically about something. Why on earth did I think logic has anything to do with anything? Why didn't I follow my heart? And why didn't anyone stop me from making such a big mistake?

This could be the first big regret of my life...