I’m looking for a word that refers to a person who has learned a painful lesson and has changed her/his conduct.
The word isn’t wiser, initiated, or chastened, but something close to these.
Context: My GF once had a tendency to pour her heart out to others, eventually got burned by several “friends,” and has now learned a big lesson and changed her ways.
Any help?
contrite - Feeling regret and sorrow for one’s sins or offenses
Penitent - Feeling or expressing remorse for one’s misdeeds or sins
pretty much interchangable, but do either words do anything for you?
I’m not sure why you reject chasten (unless you are confusing it with chastised) as I think it describes your intent.
Other than that: reformed?
Humbled.
Chagrined?
My undertsanding of chasten is perhaps wrong. I thought it meant essentially chastized.
Is it correct to use it to suggest someone has learned a lesson the hard way–and on her own?
I’d vote for “reformed”. It represents the changes made after the experience. I’ve seen too many “chastened”, “humbled”, “contrite”, and “penitent” people who didn’t actually change their behavior to put much stock in those words!
Perhaps it would be helpful if you detailed why “wiser, initiated, or chastened” aren’t the best choices. Are you looking for something soft and proud in the new lesson? or something to rub it in a bit? or ???
In the meantime, try these on for size:
remorseful
regretful
compunctious
I guess even “sorry” works
“Conditioned response”, obviously. 
To chastise is to punish, with an implication of severity in the punishment.
To chasten is to correct, with punishment (or suffering) as the mode of instruction. (Humble can have some of the same meaning.)
Here is Dictionary.com’s Thesaurus entry for Thesaurus entry for chasten.
enlightened.
Sadder-but-wiser.
Straight-doped.
I would disagree with most of the words above, because they all connote (though not necessarily denote) not merely a feeling that you didn’t something that you won’t do again, but that the thing was in some way immoral. I would agree with those words if your friend got caught shoplifting, but pouring your heart out to someone is not something to be contrite or remorseful about. You become more guarded after seeing other people’s response to it.
I can’t think of a good word in English that captures exactly what you want, perhaps cynical, disillusioned, or jaded.
Chagrined or Sadder But Wiser seem closest so far.
How about tempered (strengthened through experience or hardship; toughened)?
worldly
world-weary
jaded
Self-admonished. Self-chastened. Self-corrected. Self-schooled. Wised-up. Disabused. Disenchanted. Disenthralled. Disentranced. Embittered. Experienced. Newly-discriminating. Seasoned. World-weary.
(Also, good ones already mentioned: Jaded. Enlightened. Sadder-but-wiser. Disillusioned.)
Calm.
Embittered
Hardened
She’s learned a hard lesson
She’s lost her innocence
She’s grown up
Graduated cum laude from the school of life
My vote: Seasoned.
reformed