What kind of mental illness is it when I avoid with a passion all my messages and notifications across every app and website, I put letters marked "URGENT" in a giant pile and refuse to open them, my phone recieving a call makes me hideously upset, I have a hundred small things I need to do that would all take 5-60 minutes and yet I avoid all of them, I miss people and I crave socializing and yet every opportunity that presents itself to me feels like some terrible dread, I agonize for weeks, months, years over a message I never got back to and then send some long winded apology, re-opening contact with an old friend, only to sharply ghost them and repeat the process for another 1-5 years.
Yeouch, that is awfully relatable. For reference, I'm autistic but stuff like this can easily intersect with a lot of different things or suggest a mixture of illnesses/conditions. As PaX and Vaggumon have mentioned, your case seems to have hints (??? like food like hints of lemon, idk how to phrase) of what someone with ADHD or social anxiety would experience, it helps if you examine what the source of your behavioural patterns could be.
In the case of my autism, I have an aversion to socializing at times because I have issues of expressing myself in a comprehensible way and get tired of trying at times. I can't understand phone calls due to processing issues, too. Differs from autist to autist though, so ymmv (hope this is the right phrase).
In the case of social anxiety, your behaviour would likely have an aspect of fearing judgement to it, fear of messing up, like being afraid of using the wrong words or sounding weird when calling.
I don't know about ADHD but I imagine executive dyfunction plays a role in this? As in, you feel compelled to do a thing but your brain can't seem to divert its focus to actually do the task
So, in short, depends on how your behaviour came to be (sorry if this isn't helpful)
Edit: I took a bit of time writing this, and others have added onto this, mentioning avoidance coping and avoidant anxiety. Evidently, there are a lot of groups of origins/reasons (???) for certain behaviours, ranging form defense mechanisms to certain brain wirings, whether you're born with them or have adapted them later. Definitely seek out a professional if you can!!!