This woman is acting as if you are her on-call crisis team/unpaid counsellor, with a duty to always to be available to soothe her... This would be unbearable in a normal period of time, but when you are ill and going through chemo, and dealing with all of this in a pandemic, this is preposterous.
Her behaviour is not normal. Hence suggestions of personality disorders. (I'd suggest HPD, myself!)
You're having bloody chemo, and this is a bit of an inconvenience, interrupting usual service for her! This is surely the time to stiffen up and set some hard boundaries for this one-way-street of a friend. I would draw a very hard line: "Until I am through my treatment in X weeks/months, I am going to focus on my own health, peace, and relaxation. Therefore..."
Really, it is not your job to find her an actual paid therapist. Or to find her some other mug to unload her angst on. But therefore "I cannot be in this role of constantly reassuring you, or answering your texts, your calls when I don't answer your texts whilst receiving chemo, etc."
This must be conveyed to her, but firstly strongly to yourself. Then you have to act on your boundary, in whatever way becomes necessary. Repeat, repeat, if need be block her/pause her. Alternatively, wait until you explode one day and block her then.
It is sad, as she is someone you care about and for. But she will not change her behaviour, see she is being unreasonably demanding, or put you first. So you will have to be "the bad guy". Which is hard for nice, kind, people, but is always the way in these situations, I have found.
Your health, your peace of mind, your stress levels, these things matter, you know.