Sandra Bullock made a heartfelt plea to a judge as she seeks to win custody of her step-daughter, an adorable five-year-old named Sunny.  

INSIDE EDITION has obtained Bullock's letter to the judge overseeing the nasty custody battle between her husband, Jesse James, and his ex-wife, porn star Janine Lindemulder.

"I am aware that Janine has claimed many times that my desire to have Sunny was because I could not have children of my own. As difficult as that is for me to hear...it couldn't be further from the truth. Due to the instability in Sunny's life, bringing another child into the world at this time would not be in [her] best interest. So that is something we chose to put on hold."

Sandra Bullock and her husband have been raising Sunny since January, when the porn star was sent to jail for six months for tax evasion. After her release from prison, Lindemulder moved to a halfway house in Hollywood. She was all set to regain custody of little Sunny, but Sandra Bullock and Jesse James went to court and claimed she wasn't a fit mother.

Bullock writes, "While in Janine's care, sometimes Sunny is left alone during the day while her mother is asleep from drug use." James, the star of the cable TV show Monster Garage, said he feared that his ex would expose their daughter to "pornographers, drug addicts, guns, and felons." Her past also includes a sex tape with Motley Crue lead singer Vince Neil.

James also says his ex is unstable, sending text messages like "I won't be around. You guys win."  In court papers, Lindemulder, who has appeared in Blink182 videos, has admitted to having had a cocaine problem.

"These are very explosive allegations that a 5-year-old girl would be exposed to guns, pornography, and drugs. The judge has to get to the bottom of that. It would be negligent and reckless to give the daughter to her mother if indeed that's the environment she would be in," says CBS legal analyst Lisa Bloom.

Bullock, who is 45, told the judge, "I have been blessed with a good life...and am now grateful for the opportunity to participate in preparing [Sunny] for [her] life."

Jesse James and his ex-wife have been ordered to meet with a court mediator overseeing the custody dispute later this month.