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Ex marks the sore spot for Scarlett Johansson!

A source tells the new issue of Us Weekly (out Friday) that the 27-year-old is fuming that her ex-husband, Ryan Reynolds, has moved on with Gossip Girl's Blake Lively -- even though Johansson ended their two-year marriage one year ago.

"Scarlett is pissed that he's not under her spell anymore," the source tells Us. "She realized what a great catch Ryan was."

When Johansson briefly dated Sean Penn for five months earlier this year, she effectively halted any plans for a reconciliation.

"Ryan would have gotten back with her. He was so totally in love," says the insider," but then she flaunted Sean right after their split, and he was done."

Adding insult to injury for the Iron Man actress? Lively, 24, is sticking around as things get more serious with Reynolds.

"Things are great [for Blake and Ryan]," says the source. "They haven't had a fight yet!"