Sometimes it just happens. One of my (our) best friends for over 30 years started withdrawing from another about 8-9 years ago (but not me .... not yet), and then it became my turn. It hurts
.
Your feelings are real and understandable. Some people just change over time...
If my "best friend" calls me .... I'd still do anything for him ... we were closer than brothers for 25 of the 40 years I've known him, but he's almost totally shut down (except for an occasional call). We don't understand why .... but it hurts just the same.
Some things just "are" .... and you just move on.
Good luck!