she/he changed, started doubting you. That person started acting different, not how she/he used to.
For example, that person{best friend for 10 years) used to always look up to you, never acted detached or strange to you, always laughed at your jokes and treated you like a friend. Then, that person changes. He/she becomes more aloof, more like he/she could live without you. They act like you're not that important to them anymore.
I would, and I'd never look back.
Would you leave?
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