Something which is not terminal or fatal but lifelong

chronic

From the Oxford English Dictionary (OED)

Of diseases, etc.: Lasting a long time, long-continued, lingering, inveterate; opposed to acute

OED cites as an example:

1994 Jrnl. Internal Med. 236 685 A 23-year-old female student exhibited all the characteristics of chronic factitious disorder with physical symptoms (Munchhausen's syndrome): deliberate simulation of illness, peregrination, fantastic pseudology and dramatic circumstances on admittance. (Emphasis added)

OED contrasts chronic with acute

acute: Of a disease, symptom, etc.: coming quickly to a crisis or conclusion; of rapid onset and short duration; of recent or sudden onset; contrasted with chronic

The Cambridge English Dictionary gives as examples (rearranged):

chronic arthritis/pain

She suffers from chronic pain in her knees.

I have a chronic bowel disease

I think the word closest to this meaning is chronic. It doesn't necessarily mean lifelong, but it does imply that no cure is on the horizon. Merriam-Webster has:

marked by long duration or frequent recurrence : not acute

"Chronic" illness should work.

Historically people have either been acutely ill (an immediate but potentially curable problem e.g. kidney stones) or chronically ill, the latter meaning something that people can live with but where the symptoms cannot be substantially relieved (e.g MS or Parkinson's disease)

Recently I have heard a lot of mention of life-changing injuries. I am not sure if that term can be applied to illnesses as well.