I had a really hard time when my therapist went on maternity leave. Mine took about 4 months off, and it felt like FOREVER. I saw another therapist and I was grateful to have a supportive, caring person to talk to but it was still a very difficult time.
I had a lot of the same feelings and realizations you mentioned, especially about realizing that my therapist has a life and a family filled with people who she cares about and loves and who are much much more important to her than I ever will be (and it should be this way, but it's painful to have it so much in your face). It was also hard for me to reconcile that something so exciting and happy for her could be so devastating to me. It felt very unfair!
I don't really have advice for you, except to keep talking about the feelings and thoughts that come up around it - talk about it with your therapist, or talk about it with the therapist you see in the meantime. The only good thing is that after all of it was in the past, and my therapist was back, it made me realize that I am a lot stronger than I thought and even though it felt like I wouldn't survive, I did.
Sending you a hug, I know how painful those feelings were for me and I hope you are being very patient and kind to yourself.