I can relate to much of what you're dealing with. It's just a neurotic thought that's caught hold of you and fuels your fear.
It seems you get over one of these neurotic thoughts/feelings, then a new one pops up, but that's kind of what keeps the anxiety going.. you have the thoughts because you're anxious.. It's not really the thoughts scaring you, it's that you're anxious about something else in your life so the thoughts like what you're describing are getting to you.
What on a bigger scale in life is bothering you right now? Fear of life/existence can come down to a general feeling of hopelessness and confusion of the future. You may be lost on what to do with your life, you may no know what kind of career path you wanna take, you may be struggling to make ends meet, you may be heartbroken after a breakup, the root of the anxiety can be tied to many things.
Having struggled with this horrible, ***** problem for about 4 years now, I can tell you some coping strategies I've acquired.. each panic attack you're in feels it will last forever, that's just how they go.
Crying after the attack helps immensely, it releases those nasty stress hormones, tears rid your body of those toxins, in fact, it's kind of necessary to cry after an attack if you can.
Distract yourself with a hobby, just do something, even if you don't feel like it.
Get some antidepressant to help control some anxiety symptoms, it will make a difference.
I'm still struggling with this myself, but there are good days and bad days, you just have to work through it.