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...