By Michelle Tauber
April 26, 2010 12:00 PM

Jim Carrey was in fine form: laughing, relaxed, chatting with other guests at an autism fund-raiser at a neighbor’s home in Pacific Palisades, Calif., last month. But when he was sitting with his girlfriend of five years, Jenny McCarthy, something was off. “He was at ease, but Jenny rarely smiled and looked serious,” says a guest. “There wasn’t that spark between them. They didn’t seem to be in sync.”

Little more than a month later, the couple stunned their inner circle by ending their relationship. The breakup “was a huge surprise to even those of us who are close to them,” says a Carrey family source. “Everyone around them is sad.” Still, friends say there had always been an underlying tension between the bubbly, outgoing McCarthy and Carrey, whose manic comedy masks a darker, more withdrawn side. “She is really ‘Go get ’em!’ when it comes to publicity,” says an old friend of Carrey’s. “And Jim is nothing like that.” Another source who is close to the couple says that the twice-divorced Carrey can be “a hard guy to date.” Adds the source: “Jim can run hot and he can run cold. He is someone who desperately needs to be with someone, then just as desperately needs to be alone. But at the same time, he can be a very loving, very compassionate guy.”

In separate statements, Carrey, 48, and McCarthy, 37, characterized the breakup as amicable and expressed admiration for each other. Noting that she “will always keep Jim as a leading man in my heart,” McCarthy-whose son Evan, 7, has an exceptionally tight bond with Carrey-also said she would remain close to Carrey’s daughter Jane, 22. (Jane’s mom is Carrey’s first wife, Melissa Womer; Evan’s dad is McCarthy’s ex John Asher.) For his part, Carrey tweeted, “I’m grateful 4 the many blessings we’ve shared.”

The breakup marked a strange week for Carrey and an especially difficult one for McCarthy, who cited “different visions” when announcing the closing of the private academy for children with autism that she cofounded after her son was diagnosed with the disorder in 2005. Meanwhile, in a series of bizarre post-breakup tweets, Carrey acknowledged being “a little on edge” as he addressed everything from Tiger Woods’ wife, Elin Nordegren (“No wife is blind enough to miss that much infidelity…. Elin had 2 b a willing participant on the ride 4 whatever reason”), to his own state of mind: “This is no breakdown! It’s a breakthrough!”

But a breakthrough about what, exactly? Sources who know the famously intense actor say that he and McCarthy had slowly grown apart over time, having quietly weathered previous separations in recent years. Although Carrey has been open about his brutal battles with depression over the years, “depression had nothing at all to do with the breakup,” says one source. “That is something Jim has dealt with in the past, but he is in a really good place now.”

Another source says that Carrey and McCarthy often clashed. “Really, they are such different people,” says an old Carrey pal. “When he is not working, he wants to lie low. I was surprised they made it work as long as they were able to. Jenny can be really aggressive and ambitious.” The actress turned author and autism activist is currently at work on her seventh book-“a look inside sex and love, and how we manipulate to get both,” she told PEOPLE last month-and has an ongoing development deal with Oprah Winfrey’s Harpo Studios. (A Harpo rep calls reports that a potential McCarthy show was canceled “not accurate.”)

A McCarthy pal points to other sources of tension, noting that the star prefers to stay home with Evan while Carrey is often eager for adventure. “He likes to travel, while she prefers to stay in L.A.,” says the source. “They are both set in their ways.”

And yet, during the course of their romance, the pair merged their lives so completely that they took on each other’s passions-and children-with gusto. Carrey became a vocal advocate for McCarthy’s autism activism (see box), while McCarthy came to share Carrey’s enthusiasm for New Age spiritual enlightenment. “Their interest is to grow and expand and serve people and the planet,” says bestselling self-help guru Byron Katie, a close friend of both stars. “Jenny takes experiences and grows from them. There’s no way she can stop herself from loving Jim. She doesn’t even try.”

Several friends say they wouldn’t rule out a reconciliation between the pair-“there’s a lot of love there,” says one pal-but for now, both stars will need time to heal. “What they had was profound and real; it just wasn’t permanent,” says a friend of the couple’s. “They are both good people, and this has to be hard for both of them.”