How to regain my confidence?

I'm on the slimmer/athletic side and I never had any problem with that. My boyfriend always said he loved my body and I was very confident. Thing is we broke up recently and he started saying really mean stuff about my breasts, always implying that I'm flat-chested (saying his current gf's boobs are much bigger than mine and other things). Now I feel completely unattractive and insecure. How can I feel confident again?
How to regain my confidence?
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