Drew Gillam of AutoCloud drafted this:
Notice of termination of the <my company> Advisory Board Agreement (agreement attached)
John, I hope that this finds you well, I'm writing to notify you that <my company> has elected to terminate the <my company> Advisory Board Agreement with you. Termination will be effective as of <effectivity date> in accordance with the terms and provisions of the contract.
Per the terms noted in Exhibit A, our goal was to be able to meet as frequently as 2x per month to review our progress, discuss product feedback and features and leverage your industry experience to help us find opportunities to grow our business. We understand that you have a lot on your plate and while we'd love to have your continued support we understand that your first priority is at <Advisor's Company>. Thank you again for your support and please keep in touch.