By ELLEN GOODSTEIN

Britney Spears is drinking and smoking again, chasing away the baby blues only three weeks after giving birth. Sources say the 23- year-old, who gained 40 pounds during her pregnancy with son Sean Preston, is miserable because she's overweight and stuck in the house recovering from her C-section.

Worse still, the pop princess seems to be having trouble coping with her role as a new mom — and insiders say breaks into crying jags at the drop of a hat.

Said one source: "I think Britney is having a very hard time right now. She hasn't lost the pregnancy weight and she's desperate to get back in superstar shape and resume her career.

"She has turned to drinking wine to relax, washing away her stress."

And while Britney had quit smoking when she became pregnant, the source said she's now smoking more than half a pack of Newport cigarettes a day, hoping it'll curb her appetite and help her lose weight.

Said the source: "She feels sad, fat and is probably insecure about her mothering skills. She wants to be the perfect mom but gets easily freaked out if the smallest thing goes wrong.

"When the baby's bottle temperature isn't just right or she can't stop his crying or the diaper isn't a perfect fit, she dissolves into tears. She just breaks down crying and moans, 'I can't deal with this.'"

Britney has also been sleeping a lot — often late into the morning and early afternoon — since she gave birth on September 14.

And while the pop star is eager to slim down, doctors have told her to take it easy until six weeks after the birth.

In lieu of strenuous workouts, Britney is now watching her diet and hoping that, in addition to smoking, her new eating habits will help her shed pounds.

The pop star was notorious for pigging out during her pregnancy — but now she's refusing her usual meals of fried chicken or hamburgers and instead opting for a Slim Fast.

Revealed the source: "Her hubby Kevin tries to reassure her and tells her she's being way too hard on herself. After all, he tells her, 'You've just had a baby. Give yourself a break.'"

Britney was even more down in the dumps when her mother Lynne returned home.

Lynne had been staying with the couple at their Malibu home since Britney came home from the hospital — and friends say the pop star was hoping her mom would stay another month to help care for the baby.

When Lynne left, Britney had to hire a full-time nanny to help with baby Sean — while Kevin took on a new role as "Mr. Mom."

"Everyone was expecting Kevin to run off to party and gamble in Las Vegas by now but he's staying very close," said an insider, who added that the 27-year-old is helping prepare bottles, feed the baby and change diapers.

To help lift his wife's mood, Kevin even tried to entice Britney's family to visit. Said the source: "So far they've turned him down, saying they're just too busy to drop everything and run out to California."

That leaves just Kevin to care for Britney during her fragile state.

Added the source: "It's a total adjustment for the couple. Britney and Kevin are overwhelmed right now."

When contacted by The National Enquirer, Britney's representative said: "I don't know if Britney is smoking or drinking but she's not depressed. There's no crying or baby blues. Britney is very happy to be a mom. Sure, Britney wants to be the perfect mom. Who doesn't?"