now you do voluntary work for local community but you are unable to continue doing this job write your manager of this location

Dear Mr. ABC,
I am a volunteer of your Rajguru Nagar Local Community Services. I am doing this job for the last 2 years. I am quiet happy with my work and enjoy it the most. Unfortunately, I am unable to continue this job further because of some personal reasons which I will explain to you.
Nowadays my college has started as I am on a regular full time undergraduate course for 3 years. so, it is not going to be possible for me to continue doing this work with full dedication and conviction. As I have to complete my bachelors with good grades so that I can get admission in excellent academic colleges for my masters. I know this voluntary work requires my time but I will not be available from now on. I am happy to be a part of this noble work and will always be supportive of it. I will try to ensure my replacement for this so that this great cause will still go on strongly as earlier.
I hope you understand my position and accept my unavailability and sorry for the inconvenience.
Thanks
Yours sincerely,
XYZ