John Steinbeck's classic Of Mice and Men was set in the 1930s, and that is important context to note when considering quotes that show that Curley's wife is powerless and regretful. Women in the US in the 1930s only had the right to vote for a decade and were only allowed to divorce if they could prove abuse or abandonment. These were known as fault-based divorce laws. Women were generally not allowed to get loans on their own and were for the most part financially dependent on their husbands.
Considering this context, readers can see that any hint of unhappiness would leave a woman feeling quite trapped in this period of time. The first hint that there is trouble in Curley's marriage comes shortly after George and Lennie arrive at the ranch. The old swamper is telling them about how things are at the ranch and reveals to George that he thinks Curley's wife has a wandering eye. Here's a quote:
"Yeah? Married two weeks and got the eye? Maybe that’s why Curley’s pants is full of ants."
"I seen her give Slim the eye. Slim’s a jerkline skinner. Hell of a nice fella. Slim don’t need to wear no high-heeled boots on a grain team. I seen her give Slim the eye. Curley never seen it. An’ I seen her give Carlson the eye."
George pretended a lack of interest. "Looks like we was gonna have fun."
The swamper stood up from his box. "Know what I think?" George did not answer. “Well, I think Curley’s married...a tart."
"He ain’t the first," said George. "There’s plenty done that."
A "tart" was a slang term for a promiscuous woman or a prostitute. That's a pretty serious allegation to level at Curley's wife, but consider that the social standards were quite different then, and any woman who was boldly flirting or giving suggestive looks to men would have been a bit scandalous, let alone one who was married.
Candy has the temerity to confront her about her inappropriate behavior, including dressing and moving provocatively and always looking for attention from the men. She replies:
“Well, I ain’t giving you no trouble. Think I don’t like to talk to somebody ever’ once in a while? Think I like to stick in that house alla time?”
Candy laid the stump of his wrist on his knee and rubbed it gently with his hand. He said accusingly, “You gotta husban’. You got no call foolin’ aroun’ with other guys, causin’ trouble.”
The girl flared up. “Sure I gotta husban’. You all seen him. Swell guy, ain’t he? Spends all his time sayin’ what he’s gonna do to guys he don’t like, and he don’t like nobody. Think I’m gonna stay in that two-by-four house and listen to how Curley’s gonna lead with his left twicet, and then bring in the ol’ right cross? ‘One-two,’ he says. ‘Jus’ the ol’ one-two an’ he’ll go down.’"
This hints at the regret she feels at marrying Curley and also hints at her lack of power to change her situation. Later, she explains to Lennie why she married Curley in the first place. It's a sad story, really: she wanted to be an actress, but her mom forbade her due to the fact she was only fifteen. In an act of rebellion, she met and married Curley. She could not have known him at all, and she is now suffering the consequences of her rash decision. Here is what she reveals to Lennie:
"I tell you I ain’t used to livin’ like this. I coulda made somethin’ of myself.” She said darkly, “Maybe I will yet.” And then her words tumbled out in a passion of communication, as though she hurried before her listener could be taken away. “I lived right in Salinas,” she said. “Come there when I was a kid. Well, a show come through, an’ I met one
of the actors. He says I could go with that show. But my ol’ lady wouldn’t let me. She says because I was on’y fifteen. But the guy says I coulda. If I’d went, I wouldn’t be livin’ like this, you bet.”
Well, I ain’t told this to nobody before. Maybe I oughten to. I don’ like Curley. He ain’t a nice fella.” And because she had confided in him, she moved closer to Lennie and sat beside him. “Coulda been in the movies, an’ had nice clothes—all them nice clothes like they wear. An’ I coulda sat in them big hotels, an’ had pitchers took of me. When they had them previews I coulda went to them, an’ spoke in the radio, an’ it wouldn’ta cost me a cent because I was in the pitcher. An’ all them nice clothes like they wear. Because this guy says I was a natural.” She looked up at Lennie, and she made a small grand gesture with her arm and hand to show that she could act. The fingers trailed after her leading wrist, and her little finger stuck out grandly from the rest.
Her lack of power was due to the time period and her abusive, jealous husband. It also came because of her desperation—she married Curley in a desperate attempt to get away from her mother's rule. Her regrets are many—she regrets not being able to fulfill her dream of being an actress, and she regrets her rash decision to marry Curley, which has given her a lonely and miserable life.