I love my friend but she's emotionally exhausting.

[MacduffsMuff]

We go through the same cycle all the time. She is constantly troubled/overthinking/overwhelmed/overlooked/tense. All her own words regarding her feelings. She perceives so many things as a slight from people (usually innocuous stuff that would offend no one). She messages me, I say the same things each time and she feels better and they cycle continues. I want to help her, I really do, I know she suffers from anxiety. I just don't know what else to say or do. It's constant.

[alexdgr8]

you have been cast in the role of the endlessly patient caring parent figure.
and she is continuing to be endlessly demanding and totally self-centred.
yet she is married with children.
she behaves like this with you because she can get away with it.
why should you need an excuse not to respond to her immediately.
i suggest you just say/text the minimum to her. keep it brief and matter of fact. gradually lengthen the gaps between.
if she demands a reply, ignore/silence ring tone for her.
and when it suits you maybe reply, i am very busy and tired at the moment. all my attention is on my treatment. am conserving my energy, following docs' orders.
i wish you all the very best. don't let her drain you any more. just create space between you. don't go in for any more discussions.

[cbt944]

just create space between you

I really like that. In this space, her whirling need becomes her problem to deal with, and it may be the needed thing that propels her to seek some professional help, rather than dragging at the teat of the OP.