Long conversations with daughter

As it is exhausting and distressing for you, and frequent, I think you have the right to protect your wellbeing too.
I don't think a gentle reminder to her about how it makes you feel, would be a terrible thing to do.
Maybe don't answer the phone every time, she'll call later or you can call her and just say oh I must have nodded off. By which time she might have calmed down a bit.
Does she have friends she can talk to?

If my ACs or friends need a rant then I will listen and let them get those feelings out. I occasionally need a rant myself and glad someone will listen to me.
Let your DD rant if it makes her feel better.

Isn't that what is called being a mum? Being supportive when required?

Do you gently encourage her to see where she played a role in these situations where she always blames others.
Of course you should be prepared to listen, but she must also be open to receiving some honest advice.

She rings when she’s anxious / upset/ worried.

My two adult daughters ring me like this from time to time. One is nearly 50 the other a couple of years younger. They don’t expect me to ‘fix’ anything.

I think they just like to offload and I’ve learnt to listen and sympathise but not to try to find solutions.

More often than not events sort themselves out, fortunately.

Exactly what Cossy said.
Normally I'd say distance yourself a bit and don't always be available. But, in this case, I feel that your role is to sit quietly, making all the appropriate noises, and wait for her to have a bit of a rant. Also, remind yourself that you're lucky to have a daughter who still turns to you when she's down. Many mothers don't enjoy that status with their children - which can create an even more painful situation.
Have patience and love her that little bit more when she needs it.

Cossy is right, don't write her a letter. My DD has been like yours at times, but things have calmed down now.
The best thing you can do is let her unload if she needs to. I find it less wearing if I do something else at the same time, like knitting or ironing or cleaning the windows.
The thing she's worrying about now is me and my health! I should be grateful but I've resolved not to mention anything about that to either AC.

Would take it on the chin, a letter might feel too formal and weird to your DD who could easily take it the wrong way no matter how you word it.
As Cossy has said speak to her if you feel it is becoming too much but I also wouldn’t do this.
The danger is that she will just decide not to talk to you at all.

I’ve got a lonely friend who phones and moans incessantly. Tend to go somewhere comfortable in the house where I’ve got a view from the window or put tv on silent, then make appropriate sympathetic noises

You’re not going to change your daughter. It is draining for you but you’re her vent/outlet

We call it ringing for a rant in our family,sometimes you just want someone to listen while you get it all out, no guarantee of any solutions just someone to let off steam too

My advice, listen and have a glass or cup of something nearby, let her rant and rave for as long as she wants and make the appropriate mmm and aaaah.

There’s some on here who’ve been estranged by their daughters, make the most of yours and “put up” with her foibles.

I wouldn’t personally write her a letter, letters are too easily misconstrued.

I would gently speak with her face to face, if you really feel you must, I wouldn’t