The truth of the word
More important than being able to tell if someone is lying is being able to trust their word.
Sometimes people are very believable in the moment they are making promises to you because in that moment, they really do mean what they say. But afterwards, they act on temptation, habit, or even addiction and repeat the very thing they promised you they wouldn't do again.
So rather than trying to determine if they're telling you the truth this time or not, ask yourself if their word has been given and then broken so many times that you know you just can't trust it anymore.
A lot of times, it's wise to live in the present moment, but when you're dealing with something like this, it's important to be able to detach from the present moment and think about what you've learned from past experience.
Often, they're not "good liars" because they really are telling you the truth in that moment. It's just what changes after that which leads to them breaking their word over and over again. It's the difference between, "I believe you" and "I know I can't trust your word anymore."
Once their word has lost its value, if you want to continue with them, they will have to take further steps to not only stop what they've been repeating, but to earn back your trust. You can't do this for them and you can't even make the decision to do this. It's up to them. Things can't just go back to where they were before the whole thing started. Once damage to trust has been done, it has to be repaired with consistent effort over time.
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