Hey! Wow. I’m so glad I found this because I grew up just like you. Always moving countries and making friends and easily saying bye. As a result I don’t have “day ones”.
I think it’s understandable and okay to feel like this. It makes sense although it’s pretty sad when you don’t have that one friend that has been with you.
Other people might seem fine but I personally feel that some people who have grown up moving around a lot just shake it off Better than others.

I found your post because I was googling for the same thing. It’s not a nice feeling. I easily make friends but keeping them is the problem.