Can you seperate out "what your actions are/will be" from "you".
As an example, if your boss is disappointed, it will be because you are leaving, i.e. she is loosing your skills, etc.
So, she isn't disappointed with you... just the opposite, she's disappointed that she is losing you as an employee.
Maybe she has been supportive to you, but that was because you are a employee worth supporting. If she is reluctant to lose you, it suggests that you responded to her support and she values you as an employee. So you haven't let her down by leaving, you've met her expectaions of you and now you are bringing a successful phase to an end.
I appreciate this may sound obvious, but maybe you just need to keep reminding yourself of it.
Anger is an emotion that can sometimes be a container for sadness and disappointment, but dealing with that is her responsibility