How can I make my girlfriend honest with me?

my girlfriend lies and lies and lies ...

i caught her so many times and every time she promises that she will be honest but after a short period she lies to me again.

i never treated her bad and I never got angry when she makes a mistake and comes to me to confess ..i just want the truth nothing else ...

all I want is to know why she lies? and how can I make her honest to me?
How can I make my girlfriend honest with me?
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