I would talk to the boss rather than give him a note, too. I would try to think of more specific questions I had also. I would try to make myself "How to do X" sheets of instructions if that is what you want; "read the sheet" and write specific questions you still have and then ask those questions of whomever. Don't think about the clients you have lost being new and unsure how to look things up; think of it as now knowing some of the sorts of questions clients might ask so you can familiarize yourself with the information for the next client.

It could be that Megan needs more help than you do, not that she's so smart or able. It could be because of your age they think you can figure things out better, you have had more experience in life/this line of work? Megan could be making even more mistakes than you are.