Mr. Green, I would highly suggest you continue to search for meditation resources. You have to be persistent and you will probably read many books but the peace and tranquility of mind that you are seeking does exist. The thing about meditation is you have to be extremely patient with yourself at the beginning when you mind is most active. There is a great proverb that compares your mind as a beginning meditator to a fish, just pulled out of water, flopping and struggling in the open air. You thoughts might even more exaggerated at the beginning.
The books on meditation range from super beginner to super advanced. Keep that in mind when you are looking for books. A beginner book might be good. If you get one and it seems "to basic" then look for something at your level. Headspace is a good entry point, I think.
Also, "prayer" and "meditation" are often interchangeable words. A lot of the resources on meditation and calming the mind are religious. But, I found that certain books spoke to me at different stages of the process of learning to meditate. So, you might start with some headspace and move to some buddhist, hindu, Christian books or whatever.
My point in all of the above is to have an open mind in the kind of resources that count as "meditation" and to search out the resources that work specifically for you. Also, patience. With and open mind, patience and persistence you will learn to calm your mind.
PS I also love gardening for dealing with a need for physical activity. I then listen to meditation and spiritual books while tending to my garden.
The good thing is, as an engineer, I'm all about finding a solution to the problem, even if it's an unexpected or odd solution. Earlier this year when I was seeing a therapist for anxiety, she recommended Tapping, which seemed kinda hokey but I found that it helped break those attacks. Tapping actually looked similar to meditation to me in the way it forces you to focus on something specific and by speaking during the process, your body tunes out other distractions, creating a fairly peaceful effect. It didn't solve any problems but it stopped the runaway train.
I picked up a couple books on meditation and mindfulness at the library today. I also picked up a book by Chris Guillebeau called the "The Happiness of Pursuit."
I think I know what you're feeling, sorta like the never ending stream of consciousness that you can never turn off? The inability to be "in the moment?"
It's definitely a stream of consciousness type of thing but oddly enough it only ever happens at night as I'm trying to go to sleep. I have no problem being in the moment during the day. I've even had some fairly full days before where I still struggle with this "darkness" at night. It's like there's some subconscious threshold in my mind for what qualifies as a task worthy of spending my life doing and sometimes just being busy isn't enough to keep the thoughts at bay. It's certainly irrational to fear dying of old age and regretting not having spent my life on something I believe to be worthwhile when I'm only 33 and have decades to change that. I think maybe the thought starts there and then transforms into this thought that if I died right now I wouldn't have done anything truly meaningful with my life, which is another irrational thought because if I died right now it would matter any more. I understand that at the logical level but the brain is a powerful thing, not persuaded by logic at times.
This doesn't directly address your question about your fear of death, but one thing I was wondering that I am not sure anyone else has raised yet is how much meaningful/supportive social interaction you are getting? I think that is one of the hardest things about FIREing for many of us, even the introverts. I definitely notice patterns with my moods -- if I haven't had a meaningful social interaction with anyone other than immediate family members for more than a few days, I start to get into a funk. And it can become a vicious cycle, because the longer I am isolated, the more I sometimes feel like isolating myself, and the more funky I get, and the less initiative I have to see people, etc. etc. I didn't really experience this when I was working because even on the worst days, pretty much every day I had some kind of rejuvenating social interaction. But if I don't make a point of scheduling things and reaching out to others in FIRE, it is extremely easy to fall into a hole of isolation. Which is, of course, compounded by the fact lots of people don't really "get" FIRE to begin with, so the circle of people you can connect with meaningfully is already reduced.
Some things that have helped me are having a regular walk scheduled with my sister and friends, a book group, volunteering at the food bank, an academic group at my old university that I still participate in, and just being more mindful about putting meet ups with friends on my calendar. I find that 2-4 non-family social interactions a week seems to be a good rate for me-- more than that and I start feeling overscheduled, one interaction a week helps but isn't quite enough to keep me feeling my best.
Re: the anxiety attacks -- I had my first bout of them when I was 8 years old, after my grandfather (who I was very close to) died suddenly of a heart attack. I think the loss of that important social support figure + a certain inability or unwillingness to confront grief in my family (my parents had lost their first child to brain cancer before I was born) left me feeling adrift. I had insomnia and panic attacks for several months, until eventually I resolved it with an intense religious conversion experience. Your situation is very different, but I do wonder if the loss of meaning-making through employment + a reduced social support network might be combining here to manifest in this way.
I'm sorry the journey into FIRE hasn't been more pleasant/uplifting for you. I hope you are able to come through this without going back to work, but if not then don't be discouraged. Good mental health is important, and if work gives you that then there is nothing wrong with going back to work. But I do think that meditation/mindfulness and a stronger social network might help to put you in a better spot.
My FIRE situation is definitely not optimal, in that there are some avenues to try and get past my struggles that are closed to me right now. We're back and forth between two states, which has prohibited any attempts at an activity that is longer than two weeks. Gardening, a class, a part-time job, etc. All of those things are impossible at the moment though I expect we'll be somewhere permanently in another 3-4 weeks so that will open up some options I don't have right now.
I've been big on physical activity in the past, which can tire me to where I don't struggle with these thoughts at night. Unfortunately I have two challenges at the moment (knee, back) that have really been putting a damper on things. Thankfully I have a rower, and I'm going to attempt to make up for the weight lifting I can't do with non-impact exercise.
I think I definitely need a better social setting, and I'm hoping to work on that as well. I very much enjoy interacting with people, just not always the best at initiating it as an introvert.
I'm hoping the journal will help me see over time what is and isn't helping and allow me to make more informed choices about alternatives to try when things aren't working.
I guess in general there's just a whole lot of experimentation going on right now, and that can feel a lot like being lost some times. If anything, I'm probably a phenomenal example of why it's important to have some non-job identity/hobby/interests going before pulling the rip cord on the parachute. Thankfully, I feel like I've taken a lot of these hurdles pretty well and I'm really trying to work through them as best I can. I refuse to stay downtrodden because every day is another one I'll never get back.