I'm not sure if this is the right place to post. I feel so guilty that I have to turn this offer down in a lab that I was very excited to begin working in especially since the PI (principal investigator) has worked so hard to bring me on board and is very close to taking leave as she is about to give birth. We have gotten so far in the hiring process entirely because the PI has expedited the process as she knows I am living off of the last of my grad loans, I was supposed to start this Monday. I know she has been in a very hectic environment recently as she is trying to get everything in place for her to take leave and I just feel like such an ass backing out this far in. The job that I am taking was already offering me $7k more but I turned it down because I liked the other PI and her work so much but this new offer bumped up the pay another $13k, making it a $20k difference and thats just too much to turn down. I know that this is the logical thing to do and almost anyone would make the same decision but the guilt is really weighing on me especially since she is about to be a new mom. How am I supposed to write this email? What do I even put in the header that isn't too harsh or blunt?