There’s really no doubt about the role of stress in illness. It’s all “in the literature” as they say, even though many people seem to view the idea as a bit wishy-washy (until they have a stress-induced heart attack). If you’re interested in learning the details, try reading Why Zebras Don’t Get Ulcers. Robert Sapolsky is a great science writer and really brings a complicated subject alive. He covers everything from immunity to sex drive.
The most important thing I feel you can discover in solving psychological problems is what Able was saying about living in the moment. What’s happening right now is reality. Worrying about the future and regretting the past takes you away into a different world that may not have any bearing on this one. If you spend a month worrying about something that never happens, you’ve thrown a month of your life away and damaged your body in the process.
Learning to free yourself from the shackles of your thoughts offers complete mental liberation, and all it really takes is a little awareness. Be aware when a thought enters your mind and ask yourself if it’s useful, rational, productive etc. Do you need this thought? Maybe you’d rather let it go and return to life in the present. Your thoughts aren’t you and you can decide whether or not you want to listen to them.
Continually returning to the present in this way is the essence of meditation, and I really recommend even a basic breathing mediation to help you master this idea. In literally the space of a couple of weeks I turned from an irritable stress-head who’d want to punch my monitor every time Windows did something annoying, to someone who takes things as they come. Getting agitated by insignificant nonsense had become a health-damaging habit of mine, but it was only when I become aware of it that I realised how stupid and useless it was, and how easily it could be changed.
A book I really found useful was Rewire Your Brain. It’s kind of a gimmicky title, but it gives a good overview of current neuroscience, and you’ll see how many of the ideas I mentioned above work in practice. Meditation and other techniques like CBT are no longer new-agey hippy stuff. There’s now reams of studies to support them, so please don’t be put off by any preconceptions you may have.