If you get in a car accident, you end up in the ICU and can't get out of bed for weeks at the least. Once you get out, you have to complete physical therapy over months to get a portion of your mobility back. Once you are out of the hospital and no longer have the 24-hr support of sympathetic medical staff...

PTSD and grief! The person develops PTSD and has relevant triggers that uncontrollably illicit sympathetic nervous system activation (fight-or-flight). At first, it's only car stuff. A car back fires? Person zones out their environment, time slows down, and only nearby cars that are brightly glowing amd have the video quality sharpness set to max. Literally everything else is blurred and muffled out. The activation lasts for a while. When the person comes back, they're embarrassed. Slowly, they start cutting away people from their life as more stimuli begins to trigger the person and can't function in social settings without self-medicating. Eventually, they develop a drug problem just to make it through the day, while experiencing insomnia every night. But wait, there's more! There was a close friend with them in the car that died in the accident, so whenever they are reminded of said person, they fall into crying spell and can't function for a while as they wrestle with the guilt of feeling responsible for the loss. The insomnia and overwhelming loneliness ensures that they wake up with only half their health everyday.