I know what you mean. We get so comfortable with what we have like the people, program, campus, food, lab, colleagues, professors, and more! And now you find yourself realizing that you kind of have to start over. But I feel we have an amazing opportunity 'starting over'. We will gain new experiences, friends, colleagues, professors, food, labs, knowledge......and the list goes on. But there will always be one constant, you will always be you.
Good luck on your new adventures!