Ariana Grande and Pete Davidson hit a life-changing fork in the road last month when Mac Miller died

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Ariana Grande and Pete Davidson hit a life-changing fork in the road last month when Mac Miller died — and his death was the tipping point that led to their split … TMZ has learned.

Sources close to AG and PD tell us … Ariana was in an incredibly dark place following her ex-boyfriend’s apparent fatal overdose. She didn’t blame herself — she feels she did everything she could to get him sober — but it left her an emotional wreck.

Ariana was at ‘SNL’ Saturday supporting Pete and nothing seemed amiss. Folks who were there say they were kissing and holding each other backstage. Something clearly snapped Sunday