Why does she constantly lie?

I have been in a relationship with someone for a year -and-a-half, and am always concerned about whether she is lying to me. I have caught her out on numerous occasions and she aways apologises but doesn’t do anything to inspire trust in any way. I have tried to have conversations about this often, and she tries to explain that nothing she does is meant to harm me, but this doesn’t address the very simple question of why she needs to lie in the first place. If I cannot trust her, I don’t know if there is a future for us. She loves me a lot and I don’t doubt her feelings for me in the least, but it is getting to a point where I am thinking of this as a potential deal-breaker. I am prepared to put aside my feelings and break up with her just because of this habit. Am I being too harsh? If this can’t be resolved, should I just accept it as part of her personality and continue with this?
If you cannot trust your partner, it is a sign that your relationship is not as healthy as you think it is. You’re right to be worried, and also right to consider the future and how this will be affected by her refusal or inability to change. If she cannot come up with an explanation that makes sense, even if she points out that this won’t harm you, she isn’t addressing something that is causing real harm to you and your relationship. Tell her why this isn’t working and, if nothing changes, walk away because this may only get worse with time.

The more I know about my boyfriend’s past life, the more I want to walk away from him. Is that fair?
Everything is fair if you are placed in a position you are not comfortable with. It may be his past but if it has an impact on your present, it will affect your future with him too.