Break up with a friend

My BFF has been depressed for almost 5 years now (not diagnosed though). I am so tired of listening to her moaning about her life but I don’t know how to put a stop to this or how to help her. I am picking up the phone only out of guilt and then I feel so drained after our conversation.
I realised that she does not really want help. She just likes to be miserable and complain about everyone and everything. I can’t think of any other explanation. She says that she has no time or money to get coaching or therapy but I know it’s not true.
I was trying to find a way to be friends with her and not get upset with all her negativity. That didn’t work. I don’t think I want to be friends with her anymore. It makes me sad but I feel that I need to do this.
I guess my question is on how to break up with her. I don’t want to hurt her feelings by telling her the truth. But just ghosting her would be unfair after so many years of friendship.