I have no specific advice for your conditions. Like justcappy I thought of the self-concentration meditations as a gentler meditation for you. The opening is especially useful for your organs. You are also free to use the emptiness time however you what to direct your thoughts / qi.
As for progressing with your practice, when you have trouble doing long meditations. Well meditation is just one aspect of SFQ. Nutrition, rest, exercise and mental attitude are other elements in SFQ that you use to support your body and create balance and flow of qi in your life.
I find myself straightening my spine during the day. I can feeling the improvement immediately. This is one of those things that surprises me. Since before SFQ and meditation, sitting or standing straight was more uncomfortable than slouching. With my changed state of awareness, I become more relaxed and freer when I adopt a good posture.
I have the same experience when I adopt a good mental posture. One of Love, Kindness and Caring for myself and others.
These little redirections along the correct path make my experience of life so much nicer.
You are perfection.
Iam2