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Hey [her name], 
I hope you're doing well ! I just wanted to let you know that i've started school and it has been really both mentally and physically exhausting. Going back to in person school has been way harder than I thought it would be, and I am extremely saddened to say that i feel it would be best if I were to put my two weeks in. I would absolutely love to come back to work for you next summer, as it's been very fun and a really nice experience being here. I've loved working and it's helped me get out of my comfort zone when I was too scared to do so beforehand. Thank you so much for hiring me and letting me have this wonderful opportunity. I'm really sorry if this is an inconvenience, but I do not want my mental health to negatively impact my ability to work for you. This job has been amazing and one of the best things that I have done in years. Once again thank you so much for giving me a chance it's been an honour working here. Please get back to me when you can on what is going to happen now and what I should do. Thank you so much,

[my name]