I’ve no idea if this is the case with your friend or not but this sound like recurring depression or autism. In both of those instances, you often have to withdraw from rl interaction in order to recover between periods of time after you have forced yourself to appear “normal” when you feel far from it, which it is exhausting.
One of my dd’s is autistic and if she didn’t do the withdrawing bit she would never be able to socialise in the first place!
Scrolling on her phone in a quiet, safe, space is how she de-stresses .
I’m surprised your friend doesn’t send you a “holding” message though op as it is very rude to not answer at all.
Your friend may not want to admit her absences are due to mh reasons out of a combination of shame and guilt. Once she says it to someone in her social circle it becomes a real, concrete thing, and she can no longer pretend to herself that she is “ok”.
Look, it’s not right that you are on the brunt end of this op, and I completely understand why you feel upset, but you are friends and she has chosen not to tell you for her own reasons. It could be as much about deluding herself that she lives with a degree of normality, rather than it being about you.
I don’t know what can be gained exactly from forcing her to confess whatever is wrong with her, especially after a decade, is anything really going to change?
My advice would be to either let the subject drop and enjoy her company when she is around or to tell her straight that you can’t handle the on-off nature of your friendship, even though you have tolerated it for a long time, and end it.
My opinion here is of course totally biased towards your friend and not objective op because I know what my dd goes through in order to socialise:
-weeks of anxiety and stress leading up to event
-lots of self doubt about whether she will or will not be able to manage it at which point she may cancel
-if she doesn’t cancel, more anxiety about the journey and getting there on time and what it will involve, and if she is eating with someone else, or shopping say, is there a clear exit to the outside should she feel panicky or the shop lights affect her?
-anxiety about whether she will be able to pretend not to be affected by what are to her, overpowering smells, sounds, sights while out in public during your meet up
-anxiety about whether she will read social cues correctly and not talk too much, or too little, or interrupt, be able to maintain her concentration, and judge when the interaction is over or not and when someone says “let’s stay for another coffee” do they really mean that or are they just being kind? Or is there a hidden meaning somewhere that she hasn’t caught on to? Especially if it’s “let’s not stay for another coffee” which would lead her to think she has done something wrong,
Remember that during the above she will behaving as though none of the above is going on with her. She will appear animated, interested and humorous.
That’s not to say she won’t enjoy seeing her friend during all of the above and really enjoy the experience getting out too! Some people with ASD crave connection, but it takes so much out of them that they can only manage it occasionally,
Once back after a successful outing my dd will be filled with delight that she managed it, and probably won’t sleep much that night while all of the adrenaline drains from her body, and then she will need to recover for three days or so while she recovers her equilibrium.
Unfortunately all of the above takes a lot of energy and there isn’t a lot left over to understand how others may be feeling in relation to her. And if your friend is older, then menopause can play havoc with symptoms of ASD and she may not be able to mask as well.
This is just one theory Op. If you want to continue the friendship, and if she isn’t forthcoming for whatever reason, tell her you don’t like it but will nonetheless accept things as they are. Or, it’s completely understandable if you want to end it. If she is autistic, it won’t be the first time. And any reasonable person would understand your position too.