I want to know if ¨'' it's not my lost'' is a correct grammar.. it is pertaning to a situation where your BF had another girl then you will say,, its ok.. he dont deserve me,, its not my lost! please help me to understand this.. thank you...
You need the noun form: It's not my loss. (My loss is your gain.)

"Lost" can be an adjective: "Help me find my lost wallet."
It can be a verb: "I lost my wallet."
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I think this is more appropriate “He doesn’t deserve me. It’s not my loss.”

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