This is the first time I feel somewhat relaxed in a long, long time. What helped me? Steady-state low-intensity cardio; that is aerobic exercise. I think a big contributor to my nervous/anxious state has been that I've been training too hard. That has put my sympathetic nervous system into more or less permanent overdrive. The past week or so, I've done exclusively aerobic exercise (heart rate between around 125-150BPM), and I feel like it's significantly helped calm down my body and thus mind.
Now, I realize I'm most likely not in the same boat as most of you in this thread—nor am I suggesting that your issues are necessarily caused (or fixed) by anything like this—but I just wanted to add this as a heads up: exercising too hard can fuck you up both mentally and physically, even for a long time after the exercise. It's essential to have a balance of low and high intensity training. Aerobic exercise promotes normal function of the parasympathetic (rest & digest) autonomic nervous system (regardless of overtraining).
I read about this from some articles such as these:
Aerobic Exercise. Studies have shown that light to moderate aerobic exercise such as walking or swimming for at least 30 minutes per day at least five days per week can improve the PNS(parasympathetic nervous system) response. Over time, the PNS response and Heart Rate Variability increase and Resting Heart Rate decreases. Mind-body centered exercise such as yoga and tai chi carry similar benefits.
Turns out it *did* make a difference for me, as I can now somewhat relax and enjoy things. I no longer feel the need to fidget around constantly and punch a hole into a wall as much. My sleep has improved.