Hi Dom,
Without trying to give you any of my personal comments or another expierenced "Know It all" view point a very sensible thing you might consider is maybe look into a expierenced Chinese Medical Doctor that has the training that could do a proper assesment of you. Your body could have been developing an imbalance prior to this. They will do many exams on pulse, eyes, tongue, organ points etc.. This is a for sure as best as you can get in the Asian Medicine & if your lucky enough many are very well versed in Chinese Qigong also.
Qigong is a branch of Chinese Medicine and share the same guiding Theory's. You might want to consider contacting also a branch Instructor of YMAA, I personally recommend Ramel Rones
http://ramelrones.com he is one that I know and could give you proper training. The Qigong that Dr. Yang teaches is very safe but without a teacher you could be doing variant Mind-Body principles incorrectly. The products are a supplement, you can learn but with a correct guide you can learn safely for sure.
I have worked with many students, professional's & usually recommend "The Eight Brocades" set this is an excellent practice that is very under estimated, most only do "8" symeterical sides of each part. But can practiced 25, 50 or how many you want. I have a friend and he practices each part 100 times each part of the standing, sometimes selections from the sitting, he calls this his "800 rep wonder" and is an excellent shape and has incredible Power. He prefers to focus on simplicity and develop deeply.
Have you ever done any Sitting & Standing Meditation? This is also a very under estimated practice, I personally like this practice since anyone at any level can learn this no matter a persons health condition. You can contact Sifu Fong Ha
http://fongha.com he offers a DVD for sale & is a Master of YiQuan & Zhan Zhuang practice.
I understand your frustration since Qigong at times in the early stage can feel like such a 'Twilight Zone" subject, but I know that Master Yang has taken many steps to assist all of us in our learning. Try to connect with a branch teacher...
I hope this helps, bare min. get a teacher with expierence, if not one by you come to a Seminar or book a private lesson. Regroup your thoughts and give Qigong a another fair try maybe those sets are not right for you right now.