I have been with T for 3 years and still have the "what if she doesn't come back" thoughts when my T is away. And "what if she decides to retire while she is away".
It can make me feel panicky. I just have to remind myself that it is possible *and* it is out of my control. That these are just thoughts, nothing else. Sometimes I will humor myself with a ridiculous thought just to reinforce the idea that thoughts are just thoughts, and harmless: "What if she turns into a cat while she's away!" or something that will make me laugh.

It is also comforting to take the worry thought and think about what it means to me. It means that I care very much for my T, I care about her safety and her well-being. And it means that I value this relationship. And it means I must care about me, too, because I am allowing myself the thought of needing her and acknowledging the important place she has in my life.