What causes the feeling of suffocation during prostration (sujood) that prevents one from prolonging it, and how can it be overcome? And is the feeling that Allah will not forgive sins a whisper from Shaytan?

The feeling that Allah will not forgive is a plot from Satan. Therefore, one must have a good opinion of Allah, and combine fear and hope in their journey to Him, and perfect their deeds. As for what occurs during prostration, it might be a temporary illness, and what is important is to perform the portion of prostration that suffices.