Dear Mr XX
Thank you for your committed support to our organization in fulfilling our mission.
I am sorry to hear about your retirement and understand this would affect your support to us.
This would be a loss for us but we sympathize more with you because we know your retirement would affect you in other ways also.
Thank you for choosing to volunteer to help in one of our offices. For more information on volunteer services please visit our website or call 888888.
1. I find 'committed support' a strange collocation. (warm support/continued support/unwavering support)
2. 'support for' or 'support given to'
3. I thought retirement is not something to feel sad or sympathetic about.
4. Loss to, not for
5. an unclear and rather strange sentence
not a teacher