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A pathological liar is someone who lies out of habit. This individual sees lying as their normal and reflexive way of answering questions. Pathological or compulsive liars bend the truth about everything, whether they are big or small issues. For a compulsive liar, telling the truth is very awkward and uncomfortable while lying feels right. This pattern of lies usually develops in early childhood, due to being placed in an environment where lying was necessary. While these individuals are not overly manipulative and cunning (unlike sociopaths), they lie out of habit - an automatic response which is hard to break and one that takes its toll on a relationship. Recognizing someone with this issue will involve seeing a pattern where their actions do not match their words. If someone truly is a compulsive liar then they should not be believed.
Remember that being put in a position of being constantly lied to will become more and more frustrating over time. This is also quite disrespectful. You deserve to be respected and to be able to trust your partner in a relationship. I hope this answer is helpful. Please let me know if I can clarify further.