There was a small study (referenced below) that came out a couple of years ago which demonstrated effectivenss of guided imagery in combination with healing touch for a group of returning combat-exposed marines with diagnosed PTSD, over half of whom had guided imagery administered via a recorded CD along with healing touch twice a week for three weeks, and also listened to the CD independently at least once daily, resulting in symptoms decreasing to below the threshold for PTSD diagnosis, and by "about 14 points".

I'd be curious to know what the actual "imagery" was, but haven't been able to track down those specifics. While not directly related there was also a significant study on the very mixed results of positive self talk (also noted below), with some persons actually feeling worse as a result. With such Stuart Smalley-isms as "I'm a lovable person" having been used in that study, it seems to me a significant takeaway that what's said needs to respect a person's current state as well as their ultimate goals, so that statements aren't invalidating and making people feel worse. It sounds to me as though the "magic shield" and "i am a tree" visualizations, for you, may fall into this category. I'd certainly consider, at the very least, advising a practitioner when a particular visualization doesn't feel like a good fit, so that there is a chance to try another that may have better potential to work.

However, I don't see it suggested anywhere that the same results can be achieved using guided imagery without the "healing touch" component being included, and then there is the fact that these soldiers were provided with audio and instructions for specific regularity of use in order to be able to effectively continue on their own. Just a one-off or occasional practice of guided imagery alone may not fit the bill.



Articles referenced:

(NCBI/PubMed) Mil Med. 2012 Sep;177(9):1015-21.
Healing Touch with Guided Imagery for PTSD in returning active duty military: a randomized controlled trial.
Jain S1, McMahon GF, Hasen P, Kozub MP, Porter V, King R, Guarneri EM.

(NCBI/PubMed) Psychol Sci. 2009 Jul;20(7):860-6. doi: 10.1111/j.1467-9280.2009.02370.x. Epub 2009 May 21.
Positive self-statements: power for some, peril for others.
Wood JV1, Perunovic WQ, Lee JW.