Why can’t I trust her? Is it my insecurities or is something actually wrong?

I’ve been dating my girlfriend for a year, and we’ve had our ups and downs. I caught her lying about hanging out with her ex and not telling me about it. She says she didn’t want to argue, but how can I trust her after she lied for a month? Should I give up?

Why can’t I trust her? Is it my insecurities or is something actually wrong?
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