Why does she keep lying?

My girlfriend has a habit of lying whenever she is trying to get away with something she knows I don’t like. She lies when she’s going out to pubs with her friends, or if she has to meet a friend and it clashes with our dates, and this usually doesn’t bother me but it makes me nervous. We have been together for two years now and I love her a lot and want to ask her to make this relationship formal. I don’t feel safe enough to do that only because her lying makes me feel like she doesn’t trust me or, worse, that I can’t trust her completely. We have spoken about this many times and I have always told her how it makes me feel, but she doesn’t change this habit. If it continues, I don’t know if it is more sensible for me to walk away from this relationship. What should I tell her?
If she acknowledges how you’re feeling and does nothing to change, it feels as if she isn’t taking this relationship as seriously as you are. Trust is critical, more so if you intend to evolve as a couple and are considering a longer commitment. If you have mentioned this repeatedly and she knows it hurts you, but keeps repeating this behaviour, you may have to reconsider what your future with her is. To move on without trust is simply to delay the inevitable. You can try talking to her about it one last time, but how much longer you are prepared to wait for progress is entirely up to you.

I want to ask my boyfriend to stop being passive aggressive towards me whenever we fight, but he doesn’t even believe he is being passive aggressive. How can we fix this if he doesn’t even think a problem exists?
You can’t, and he needs to know this relationship is suffering if he isn’t being honest with himself or with you. Consider speaking to a third, neutral party, because that may help.