It is understood that retrograde planets indicate the challenges of life or even highlights for success and growth, depending upon their placement and association. There are diametrically opposite viewpoints regarding the results of retrograde planets in different texts. How they will behave is debatable, and one needs to learn with experience.
However, I agree with you, and experience shows that retrogrades indicate a need to revisit the past. Though it is a difficult to fathom that one soul needs to have multiple revisits having all retrogrades, another one does not have to look back. And it is not that the latter one is not facing challenges. Anyhow, that is besides the point under discussion.
Besides the ‘unfulfilled desires’, the DRIDH PRARABDAHA signified by a retrograde could be the karm phala of a past action. These phalas could be moare specifically related to diseases and sufferings, than a happy and peaceful life, which direct planets give anyways. Our Puranas, often condemned as stories or exaggerated treatises, have deliberately laid stress and given detailed descriptions, with examples, of the Karmas and their consequences. It may not be just a shrewd psychology and insight being put to a practical use.
Till the soul attains self-realisation and gains absolute knowledge, there is a constant ebb and flow, till a final divinisation of the individual happens. Until this state is attained the human being swings between the sublime and the bathos. His two different aspects react to the external stimuli in their own distinctive manners to take the soul forward. There is a tug-of-war between the animal and human natures to gain precedence and dominate over the field of jiva-consciousness and hence the different good and bad karmas. In this context, Puranas (specially Garuda Purana) have quoted specific “Phalas” of sinful actions and crimes, which result in specific diseases in the next birth.
This could be one of the reasons why retrogrades associate with diseases. As per puranas, most of the bad actions lead to bodily suffering, by which man learns, it seems. E.g, one who plugs up or blocks up the hole of a rat or a snake, who catches fish and causes them to die by suffocation, who stifles the life of any creature, gets asthma, lung diseases, pleurisy, bronchitis (severe), pneumonia, etc. That might be a retrograde Saturn in Cancer?