Hi, Lee! Is this editor position causing you a lot of stress? Do you enjoy the "job"? Since no one has said anything to you, you are probably doing an excellent job! Even though you have forgotten some things, you probably are doing a better job that someone without fibro would do. They would let you know if there was a problem.
I have found that nearly all of us are Type A personalities and are perfectionists. This could be playing into your thoughts. You may just be hard on yourself. So, if you really enjoy being the editor and are getting the job done, stick with it. If you are really stressed doing this work, then resign. BUT, don't let fibromyalgia rule over your life. Do what you REALLY want to do. You can always work around fibro.
Let us know what you decide.
Sherrine