Let he go and get on with life
There could be some possible answers to that.
1. She's a natural-born-liar. Means that she lies to everyone. Not just you.
2. She thinks you are easy to be tricked and once she finds out you are, she keeps doing that again and again because she thought you would never find out.
3. She has no loyalty for you. Means that she finds it unnecessary to be honest to you. She could possibly think you're not worth it.
4. She's hiding something from you. It may be something that you could be very mad at, or something she's very embarassed about.
The thing is you have to try to gain her trust, by showing how a dependable person you are, and how you're never gonna judge her for everything she has or gonna decide.
If you have done so, but she keeps on lieing to you. Leave her man. She's not the one.
I support number 1