Thanks for giving me such a thorough response and offering your insight.
I completely agree about it being difficult to make clear decisions when your head is a mess, and that I don't have to make any final decisions about my future right now. However, I'm so burnt out right now, and have been doing so little with my work that it's currently not possible to back off the accelerator anymore at this stage.
I agree that I need to work on myself for a while and make my health my priority. My biggest problem, as you say, is living my life in my own head, which I have been doing for many years.
I definitely need to get to the root cause of my problems as I don't want to get into middle age and realise I've spent my entire life ruminating on the past or worrying about the future. I've been practicing buddhism, meditation, CBT/stoic principles etc for a while now and attempting (rather poorly) to live in the moment, but like you said, this isn't going to be a quick fix.
I know I can't just run away from my deep problems, I know they'll continue to follow me no matter what I do if I don't dedicate the effort necessary to 'fix' myself. However, I think pursuing a career that encourages me to relax would be killing two birds with one stone and I'm toying with the idea of getting certified as a freediving instructor, which in itself requires a lot of yoga and deep relaxation exercises.
How did you feel when you finally realised you had to change and slowly back off from your career in engineering? I'm guessing it wasn't an easy decision to make. Did you feel relieved when you finally decided? Did you feel angry at yourself for having 'given up' on something that must have presumably been a passion of yours at one point and have required a number of years working towards a specific aim?