Good Lord, did you say this girl was a psychotherapist?? She sounds like a very sad case who needs one herself. (Actually, I guess alot of them do!)

Friendship is priceless and can go through
tough times. If you feel you've hurt her, it's worth an apology for tone of voice, but not for realizing that you have different needs. Maybe you can cook your favorite Italian dish and she can bring something (shared responsibility sounds like an important lesson for her!) If she's not aware,you could tell her of a reputable tour companies which welcome single travelers.

I feel for you both, and hope that your trip is all you've envisioned. For me, vacations are a time to recharge and explore. I'm wondering if either would have happened had you traveled together.

Take care.