As already noted just list the end date, the reasons and keep any emotion out of it. Do not apologize in any way.

In person if they are trying to convince you to change your mind etc the most you should do is note that you recognize the inconvenience it causes them but note that they have not been on board with helping you and therefore care cannot continue. No apologies or anything (that will just give them the idea you are in the wrong).

Best of luck.