I'm currently going through this and something that has helped me cope whenever I feel myself on the brink of an anxiety spiral is really just trying to remember that the people I hold closest in my life are the last people I need to worry about hurting me. While friends are often "cast aside" when it comes to romantic relationships, I think a common theme regarding close/best friends is that friends will never leave you in the same ways that romantic partnerships end. The closest friend will always be near me when I need her most because that's what friends do and at the end of the day, I really shouldn't worry about it.
And say, if they do leave you...well that tells you what kind of person they are and what kind of friend they are too. However, with that logic, if you truly know someone well, you should also know them well enough to know they would never do that.
In terms of keeping in touch, just honest communication about how friendships mean practically everything to you. I think it reaffirms to your friend that your relationship is important and a commitment.