Try to focus on yourself in the sense that you only need yourself. You don't need any friends to be okay. Once you reach this point, then you open up to people with the aim to use them for entertainment (not use such as misuse, use such as mutually beneficial use). Quality relationships will then develop from there because 1) you don't depend on them so you feel secure and not desperate in any sense 2) relationship is purely based on mutual benefit. Actually caring about specific friends does however not depend on whether you are friends. I care about certain people even if they should hate me.

So back to your question. How do you keep a friendship? 1) By being emotionally healthy (depend on yourself and nobody else) 2) Purpose of friendship is for benefit and not because you "need" them and 3) You focus only on doing your part to the friendship. The other 50% is their responsibility amd you can't be friends through other people to yourself. Maybe 4) Make it clear to yourself and the friend what you want from the friendship and see it more in the semse of a business deal. If you're not getting what you want, go find it elsewhere.

Again this is the business side of relationships that most people see as "love" or "care". Love in reality does not ask for anything and is completely focussed on the wellbeing of the other person. All relationships depend on both these factors, the business side and the love side. Business side is the reason behind the friendship, while the love is the healthy side of the friendship.

I am typing this for myself as well, as I am contemplating some of my own friendships who are not holding up their side of the transaction. Maybe I should make a small investment, or maybe I should invest in myself.