The 47-year-old actor split from Megan after nearly 10 years of marriage in May, and he's ''not taking it personally'' that she's already dating the 30-year-old rap star.
A source told Us Weekly magazine: ''Brian isn't devastated by their split anymore, as they have been through break-ups and splits a few times now.
''He doesn't care that Megan and Machine Gun Kelly have been flaunting their relationship publicly and is trying to focus on his own life and obviously dating a few girls in public himself. He's not taking it personally.''
The insider revealed that it was Megan, 34, who ''cut the chord'' on their relationship, adding that Brian is grateful to have moved on from the ''constant drama'' in their marriage.
The source said: ''Megan cut the chord with Brian and was just over it.
''It had to with her meeting Machine Gun Kelly, but both of them know it was finally time to call it off. Brian is happy and is enjoying not having to deal with constant drama.''
Brian previously revealed that he was surprised at the amount of Instagram messages he'd received after his split from the actress.
Speaking last month, the former 'Beverly Hills, 90210' star said: ''I had no idea that Instagram was so much of a dating site until I was single.
''Up until then, I posted pictures of benches, trees, the ocean and my kids doing stuff here and there. I was a total married dad on it. All of a sudden I started getting DMs when Megan and I separated. (The DM's) started flowing in instantly, no shame in their game at all.''
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