She still looks great, just older and quite '70's.
Remember she smoked, took sleeping tablets and I'm guessing other prescription drugs.
She lost one of her brothers when she was 20, lost her baby girl, and was hounded by the FBI until her death.

Here's a photo of Jean with director Raoul Coutard taken in early August 1979, just weeks before her death. Seberg was hired for a supporting role in Coutard's film "La lgion saute sur Kolwezi" and completed one day of location filming in Guyana. She was scheduled to continue work the following month in Paris. After her death, she was replaced by the American actress Mimsy Farmer.
http://www.philippelegouis.fr/philippe_legouis/jean_seberg_et_r.c._la_legion_saute_sur_kolwezi.jpg

The contrast is alarming.
"Did you make coffee? Make it!"Cheyenne.

No one looks at 40 as they did at 19 (!)
When you consider how much different she looked just 9 years earlier, in Airport, she had clearly aged very rapidly in a short time. This wasn't just natural aging.

It's possible you're right, as well. I don't mean to dismiss your claim entirely. Frankly, I had forgotten she had drug use at the end. When did that start.when her daughter died?
Sorry I was unclear.
.

My thought the reason she looks this way in the 1979 photo, is because by 1979, years of drug abuse were catching up with her.

Well, she's still a beautiful woman, either way.
A great deal of the difference is one is a carefully posed studio portrait that would have involved the craftsmanship of many people and thousands of dollars, and the other is just a snapshot taken in natural light.
No one looks at 40 as they did at 19 (!)