The only thing I can think of in this horribly difficult situation is your suggesting that your fiance hire for his mom, not a therapist, but a personal organizer. The mention of therapy quickly becomes interpreted as a desire to lock her up in a mental health facility, but the mention of a personal organizer, done in an upbeat way, would not conjure up such a negative image.

As has been suggested above, you need to tread very carefully and consider your own plans and your own long term stability. What if this woman never changes? What if things get worse with age? The latter is probably a more realistic assumption...

When your fiance again mentions that you come from a good form, and thus would never understand, respond with saying that the issue is not that his mother's status is not equal to that of your parents, but that she has been exhibiting severely abnormal behaviors for years if not decades, that these behaviors cause her extreme anxiety over making the ends meet, and that she might be helped with a professional intervention.

I am sure that professional personal organizers have seen it all, including abandoned storage units. So nothing to be embarrassed about.

Another thing is to ask who is paying for the storage units.