Rebecca Minnock, 35, went missing from her home in Highbridge, Somerset, with son Ethan for 17 days after a court ruled he should live with his father.
Writing on Facebook, Ms Minnock said she had endured "two years of hell."
She handed herself in to police in Oxford on Friday following pleas from police, family members and the courts.
The pair vanished on 27 May - the day Ms Minnock was due to attend a family court hearing in Bristol and hand her son over to ex-partner Roger Williams, 39.
Mr Williams, of Burnham-on-Sea, confirmed on Saturday that he had been reunited with Ethan, and said: "Ethan is safely back in my care now."
Avon and Somerset Police have made no arrests since Miss Minnock came forward.
In Ms Minnock's social media message, she thanked supporters who had signed a petition urging social services to re-examine the custody case.
"That heart-breaking moment when you see that the little seed your baby planted weeks and weeks ago has finally started to grow, and he's not here to see it," she wrote.
"To say I'm devastated is an understatement, I have today read all the support from all of you and am speechless.
"I am preying (sic) that this case gets re looked at properly and the judge can see that my son belongs with his mama who absolutely loves him to bits.
"Think you all know how much I adore Ethan. Two years of hell, but would do it over and over if it meant he was safe with me. Xx"
In another post on a separate page, Ms Minnock wrote: "To all the mums and dads fighting for their children: please keep going and never give up.
"I will never give up on keeping my beautiful baby safe."
Judge Stephen Wildblood QC, the judge dealing with the case, confirmed a family court hearing would take place at 10:00 BST on Monday at Bristol Crown Court.
Ms Minnock's mother, Louise Minnock, of Highbridge, and her partner, Andrew Butt, of Cheltenham, were sent to prison on Friday in relation to the pair's disappearance.
A mother who went on the run with her three-year-old son for more than two weeks has said she will never give up on trying to keep her "beautiful baby".