"She's happiest when they're together," the actress's trainer, Christopher Lane, tells PEOPLE. "She's thrilled to be with the man she loves."
Bertinelli, 50, and Vitale, 48, who got engaged in March while vacationing in Italy, are "going to be a great married couple," says Lane.
"I don't know if a whole lot is going to change. They still really like each other," Lane says about the couple, who met in 2004 after their previous marriages ended. "They adore each other. It's not easy to mix families and they've done such a good job."
In addition to making Bertinelli laugh, Vitale has been a strong support system for the actress during her weight-loss and fitness journey, according to Lane, who has been training Bertinelli since November 2008.
Case in point: Vitale ran the 2009 Sonoma-Napa half-marathon with his girlfriend and rode his bike alongside Bertinelli while she trained for the Boston Marathon.
"I've never felt more beautiful at any weight than when Tom looks at me," she told AARP The Magazine recently. "He loved me no matter what. In his way, he's teaching me to love myself just as I am."
Reporting by ELIZABETH LEONARD