I would be equally hurt and devastated to find something like this from either of my DDs, for whom I have done untold good things since they became adults. However, I think the best advice is to say nothing about having seen the note and try to ignore it and carry on for a while longer before winding down your involvement in her business, such that she will not suspect that you have found it. Much depends on whether you think you could do that after reading the note, without bursting into tears - as I might do. In which case I might just say that I am not feeling well so am going home early, and follow up a few days later to tell her you don't think you are able to carry on in the job.
If you say you have seen the note it could cause more upset and argument with possible permanent consequences. As others have said, she might have just been at a particular "low" on the day she wrote it and said more than she really felt. After all, if she really thought you were so incompetent and the rest she would probably have said those things to your face, so I suspect she did not really mean them.
Be the bigger person and even though hurt don't burn your bridges for the future.
Maybe later you might even refer to some of the things she said such as "I never really felt that I was good at the job I did for you" or "I always struggled with such-and-such", and try to forget all about it.
I'm sure you won't find that easy to do - and neither would I - but you would gain nothing by taking any other approach.
Unite the Kingdom and Pro Palestine marches Cup 16th May 2026
