JUSTIN TIMBERLAKE & JESSICA BIEL: HEADED FOR HEARTBREAK .....

Justin

Jesica

Jessica Biel finally got her wish — after a pre-Christmas proposal, she's on her way to being Mrs. Justin Timberlake. But insiders say the union may already be doomed. "Jess has told her friends she knows he's got a roving eye but she's prepared to look the other way if he should ever stray," an insider says. "She's in it for the long haul." Justin, 30, has assured her his womanizing days are over and even promised to delete girls' numbers from his phone. Jessica's going into the marriage hopeful, but a realist. "He's been linked to so many women, from Olivia Wilde to Mila Kunis and even Ashley Olsen. She knows she has to take the good with the bad. He might have every intention to not cheat, but he may not feel the same after a few years and she's aware of that." Still, while her pals fear the worst, one insider says that Jessica, 29, may be stronger than she's given credit for. The evidence: how she handled their brief breakup. JUSTIN TIMBERLAKE SHOWS OFF A FULL BEARD — LOVE IT OR LOATHE IT? "Jessica's played a cool game," says a pal. "She's a big fan of Kate Middleton and saw how she remained aloof and kept quiet after Prince William dumped her, and now, she's a princess! Jessica did the same thing — even though she was absolutely distraught. She played a very successful game and has finally won her man." Still, friends fear Jessica is setting herseulf up for heartache. Justin seems to have all the power and is "treating Jessica like a doormat," an insider says. "Often, a week will go by without seeing him. She's even suggested he teach her how to sing so they can spend time together. He's calling all the shots. But she's taking what she can get — and the one thing she wants is to be his wife, at any cost." Newly Engaged Justin Timberlake Shows Off a Full Beard -- Love It or Loathe It? Justin Timberlake & Jessica Biel Are Officially Engaged! Did Justin Timberlake Propose to Jessica Biel? ** Justin Timberlake's Marine Corps Ball Date Says He "Brought Sexy Back" to the Dance Floor **