“Regress” vs. “retrogress”

Question

What do each of them mean exactly? Is either (or both) the opposite of "progress"?
Could someone please explain the difference?

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To add some context:

When I look up the definitions I see the definition for regress is "to return to a previous, usually worse or less developed state", whereas the definition for retrogress is "to go back to an earlier, esp worse, condition".

They both look the same to me: antonyms of "progress".
But when I used the term "retrogress" at one point someone corrected me to use the word "regress", and I didn't understand what the difference is... so now I'm confused at how the two are different, and when to use which.

Answer 1

In normal usage there isn’t much difference between the two. There are eight main definitions of the verb regress in the Oxford English Dictionary against three for retrogress, and all three have cross references to various definitions of regress.

The Corpus of Contemporary American English and the British National Corpus show that regress is much more frequent in both American and British English. Unless you have a very good reason for using retrogress, your language is more likely to be effective if you use regress instead.

Answer 2

I am not sure but the below sentence(taken from - "A Grammar of Freethought", Chapman Cohen), i think antonym of progress is retrogress.

At one time we call its consequences moral or progressive, at another time we call them immoral or retrogressive, but these are some of the distinctions which the human mind creates for its own convenience, they have no validity in any other sense.

From an online thesaurus, its found that

Main Entry: regress

Part of Speech: verb

Definition: return to earlier way of doing things

Synonyms: backslide, degenerate, deteriorate, ebb, fall away, fall back, fall off, go back, lapse, lose ground, recede, relapse, retreat, retrogress , revert, roll back, sink, throw back, turn back

Antonyms: develop, grow, progress

so antonym of regress is also progress.

Worsen or to sink or to collapse is retrogress. To diminish or to decay is regress.

In the novel, the survivors of a nuclear explosion retrogress into a state of barbarism and anarchy.

And while many autistic children show signs of autism from birth, some seem to develop normally but then regress after their first year.

But when to use these two, is entirely depends on the context, i think.