I'm not being disingenuous.
Perhaps I wasn't clear though - most of my comments have been about friendship more generally, and to some extent what I think your friend wants, rather than this specific situation.
As far as the specific situation goes, I do think it's uncalled for to not respond within, say, 24 hours, to a message asking how you are. You could easily reply in a few seconds, saying you're ridiculously busy but you hope she's closer to resolving XYZ thing you spoke about. Written in a kind tone.
It was a bit of a massive drip feed to mention you'd actually had an in depth conversation or two not long before!
I feel for your friend though. It seems clear it's you that has changed, meaning she has lost a good friend. It is always sad to lose a good friend, and obviously contributes to feeling alone. I do wonder what it is that has made you change. Unfortunately some people seem to drop friends, or at least not really enagage, once they are in a romantic relationship. (Then come crawling back for support when they are single again!) If you have been having in-depth chats I'm not saying this is you, but something has made you step back a bit. Maybe just lack of geographical proximity?
You now mention that you are "constantly sharing your thoughts and swapping thoughts back and forth with the people you live with, local friends, partners, family", but previously made it sound like you only share in this way with the people you live with. If you're keeping up a decent sharing community and many relationships outside the family that's great, and precisely what more people should be doing!
You don't need to have that level of friendship with everyone. But as I said above - she was clearly one of the few you did choose to share like that with.
Agreed it's crap if her communication drops off when on holiday! The sort of sharing friendship I have written about would mean still sharing whatevers going on. Could you perhaps use this to explain to her why you can't always reply in depth (but do reply briefly!)
I feel for her though. Even if it's absolutely completely reasonable for specific friends to become distant or fall by the wayside ... it's horrible to feel alone. Every single one could have a totally understandable reason for disappearing, but it doesn't stop the resulting situation being soul crushing. Please have compassion for that, at least.