Huffman, who along with fellow actress Lori Loughlin are among the most high-profile parents to be caught up in the admissions scandal, will be sentenced on Friday.
"In my desperation to be a good mother I talked myself into believing that all I was doing was giving my daughter a fair shot," Huffman said in the three-page letter. "I see the irony in that statement now because what I have done is the opposite of fair. I have broken the law, deceived the educational community, betrayed my daughter and failed my family."
Huffman's letter adds yet another dimension to the sentencing decision that will await Talwani.
Prosecutors have suggested one month in prison and a $20,000 fine for Huffman.
Her lawyers are asking for no jail time, one year of probation, 250 hours of community service and a $20,000 fine.
In a motion filed Wednesday, Huffman's defense team says the government's sentencing memo cited examples of previous cases that are "very different" than hers, and any comparison is "apples to oranges."
Huffman's lawyers argued that the sentencing guidelines in those cases were much higher and reserved for the mastermind of schemes, not mere participants.
Prosecutors, meanwhile, have argued that universities and testing companies were the victims of the schemes, and the amount that parents paid should correspond to the penalties' severity.
Of those charged, more than 30 are parents, accused of conspiring with Singer. More than a dozen of those parents, including Huffman, have pleaded guilty to conspiracy to commit fraud.
Huffman is expected to be the first parent sentenced in the case.
So far, only one person has been sentenced: Former Stanford sailing coach John Vandemoer, who served no jail time.
Huffman has written that before she paid $15,000 to rig her daughter's SAT test, she worked with Singer's college counseling and preparation business legitimately for a year to try to improve her eldest daughter's math SAT scores.
She wrote that Singer eventually offered to have a proctor boost the daughter's scores after she took the test, without the daughter ever knowing. Huffman struggled with the offer for weeks before relenting, she wrote.
Huffman recalled the moment her daughter "looked at me and asked with tears streaming down her face, 'Why didn't you believe in me? Why didn't you think I could do it on my own?'"
"I had no adequate answer for her," Huffman wrote.
Singer pleaded guilty to four charges in March related to cheating on standardized tests and bribing college coaches and administrators.
Huffman's husband, actor William H. Macy, and other loved ones submitted letters to Talwani on Huffman's behalf, pleading for leniency.
Each letter outlined Huffman's loyalty and commitment to the writer -- an attempt to show her role in the admissions scam was an outlier in her life.
Macy also shed some light on the immediate aftermath of Huffman's early morning arrest by the FBI in March.
Two days after the arrest, Macy wrote, their eldest daughter was headed for an audition at a school she had her heart set on -- one that didn't require SAT scores. But when she got off the plane, the school emailed her, formally rescinding its audition invitation because of the scandal.
Macy wrote their daughter "called us from the airport in hysterics," begging them to "do something, please, please do something."
Macy also said his daughter still was traumatized from watching her mother's arrest.
Eva Longoria, Huffman's former "Desperate Housewives" castmate, also wrote a letter, saying she remembered Huffman always helping with Longoria's charity aimed at helping poor Latino children.
And she especially remembered Huffman's help during contract negotiations. Longoria was set to be paid less than the other main performers, until Huffman stepped in and made them all negotiate for the same pay, she wrote.
"It wasn't about the money for me; it was the fact that I was seen as an equal, which is how Felicity had always seen me," Longoria wrote.
CNN's Eric Levenson contributed to this report.