what shall you do when your optometriest whom you've been shadowing for half a year asked you to write yourself a LOR and she will revise it and sign it?
write a great one and let her edit it! a lot of people feel moral ambiguity when their own optometrists asks them to write their own letter..but if they are ok with it, what is the problem? Ive never been in that situation yet, but I've known someone who did that and got accepted into Waterloo.
Negative: the letter will sound like every other essay you've written due to style/voice. also, you will miss the opportunity for someone to offer a different angle on your talents. it won't offer the admissions committe any more insight on who you are because you would have already given it to them in your essay.
Final answer: in the end it won't matter that much. most letters are all shiny and golden. so long as your LOR does not have any red flags like "this is not a people person/would not be great with patients/i dont know why they asked me to write this letter" then you should be good.
