How to tell a professor you've changed your mind? I need some help writing a polite email to a professor from a different department at my university. A while back I emailed him because I was interested in doing graduate studies at his department (I'm at a department in a pretty similar field). I asked him about the entrance exams and all the subjects I would be required to take before officially applying to do graduate studies. He answered back very politely recommending me some courses and also telling me which courses I would be required to take before officially applying. Unfortunately, it was far more than I'd expected, and I realized I'd have to do an extra year of undergraduate studies in order to do it.

Since I'm currently in a field that's very demanding, I think I wouldn't manage to pass all those extra courses (there's more than 10), and would probably just end up having a burnout. 
Now I don't know how to politely and considerately tell him I appreciate all the effort, but that I've changed my mind and I won't be applying to do graduate studies at his department. I feel like I just wasted his time because he seems to have put a lot of time in this, discussing it with his colleagues at the department and answering all my questions shortly. 

I should say I'm at a small and not at all famous university in Europe, and they probably don't get many admissions or questions about graduate studies...
As others have said, keep it short and straightforward. Like this:

Dear Professor,
Thanks for all the info, I really appreciate it. Unfortunately, this list of courses is far longer than I expected. Given my other commitments, I'll probably not be able to pursue this further. Still, your response was very helpful in making an informed decision, thanks for taking the time.
--Name