The Zen Miracle Cure for Anxiety & PTSD
It's not for nothing that in old Japan -- where life was often nasty, brutish and cut short -- Zen was the "religion" of warriors.
Doing Zen in the samurai way can resolve your anxious mental turmoil and keep you from being "stressed out" by anything at all.
The basic idea is to start with some attention to breathing and body posture. Learning to sit in Seiza, that's an excellent start (see the diagram above).
Assume the Seiza posture and rather than either thinking or imagining anything or directly trying to stop thinking or imagining anything, keep bringing your attention back to your breathing.
Now, for some people "breathing" is too diffuse and it's impossible to really concentrate on that, so in Koichi Tohei's "Ki training" we're taught to focus on a small area just about two fingers below your navel and about one inch inside it.
Note that the reason this kind of "one point" mind training in meditation works is that it brings the awareness, energy and attention down from your upper body where it's getting knotted up as "thoughts" and very uncomfortable emotions (in the neck, jaw, head, and shoulders especially) and constricting the circulation of Ki.
Each time the attention wavers, which it will, you gently bring it back to the "one point."
If even this is difficult at first, there are two even easier methods (I actually use them all):
The first is to light a candle (a small votive type candle in a dish or a glass is best so you don't get distracted by dripping wax) and focus all your attention on the flame of the candle as the "one point," getting completely absorbed into it by bringing your attention back each time it begins to wander into thoughts or images of the past and future, any worries you might have, &c.
The second is to use a small indoor fountain that has a stream of water running very quietly over pebbles. This can be very absorbing. It's the same idea: you listen to the trickling water (while sitting firmly yet comfortably in Seiza) and the changing, subtle sound of the water is your "one point." Each time your attention begins to waver, you gently return it to the sound of the trickling water.
Seiza is by itself a powerful and stable posture, and even more so if you completely relax into it by dropping your energy downward -- always downward to the Hara, to the lower body, the knees, the feet, the hands (assuming they're resting on your knees or making a mudra).
Always drop Ki downward, because in anxiety attacks the body's energy rushes upward and overloads your head (or, even worse perhaps, gets choked and knotted up very painfully in the chest and/or throat).
By using Seiza along with "one pointed" concentration you can overcome any anxious thought/feeling.
Better still, you will soon directly realize that the nature of the mind is infinite space.