She barely stood at 5 feet tall and was round, with hair as white as snow. However, every day
spent under her care co me lunchtime, the most dreadful part of the day,
When outside of my home and away from the customs my mother
implemented experienced, I came in contact with an unknown world of strange customs and an unknown language. Adapting to a new culture took tremendous courage and I feel fortunate to be in a melting pot of cultures and to be able to experience the best of both worlds.
I thought you addressed the prompt adequately using the mayo sandwich metaphor. However I felt that there was an abrupt conclusion/reflection of how it shaped who you are today after you talked about the mayo incident.
You could shorten the part about the description of the nanny, and I thought starting the essay with mayo would be a great hook. For example, you could start your essay with "No! No! No!" (in reply to all the ingredients your nanny offered).
I guess you need to explain better the link between mayo incident and the difficulty of adapting to a new culture. It could be that you're always adding mayo only and afraid to try other ingredients, you too were afraid to assimilate into the new culture because you were so used to your previous ones. Of course at the end you need to show the readers you were able to reconcile that.
I hope this helps!