Long conversations with daughter

Whenever anything goes wrong in my adult daughter's life or things don't go her way she always blames someone else. She phones me and wants long conversations constantly going over the same worries and problems.
I find it exhausting and it leaves me feeling distressed. I try to be supportive but there is only so many times that I can listen to her moaning, I try to cut the conversation short but then she says that I am not interested in and she would expect that as her mother I should be prepared to listen.
This problem often results in me being pushed so that I sometimes say things that I don't want to.
She can be the most delightful, caring loving daughter when something is not bothering her.
We have fallen out many times over the years and I would like to change change our relationship so that this doesn't happen. I am thinking of writing her a short, gentle letter explaining how I feel, I would welcome any advice.