I don’t know how to tell my longest friend I do not want her to be my maid of honor?

My friend of 18 years has recently been diagnosed with bipolar polar disorder. She’s been having a really rough few years and I’ve been trying to help her as much as I can. To make a very long story short this is all starting to take a toll on my mental health and I’ve distanced myself a bit from her. I still keep in touch and see her occasionally but I can’t be involved as much as I used to because of the things she has done and how it makes me feel. I’m going to be getting married and we have told each other since we were 11 how we’re going to be each others maid of honor one day. I have another friend that I have been very close with for about 15 years and she has been a much better friend to me in recent years. It’s not that I don’t want my other friend to be my maid of honor it’s just she’s really lost herself and I don’t think she can handle it. I’m not a bridezilla in any way shape or form. I don’t like fancy things and I certainly don’t go to fancy places. I’m nervous about how she is going to behave like, giving a speech etc. She talks to herself and says she’s talking to Annuki gods for crying out loud. I’m just torn. I love her but she isn’t the same person that I used to be able to count on anymore and I don’t know how to break this to her without hurting her feelings. That’s the last thing I want.
I don’t know how to tell my longest friend I do not want her to be my maid of honor?
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