I'm going to break it off with the girl I'm seeing.
What's the reason for breaking it off with her?
I found out she's been lying to me and I don't respect her anymore.
Lying to a person is never a good thing.  It is okay to end a relationship if the trust is gone.
You're right, trust is very important.
I think it would be good if you let her know why you're ending the relationship, so maybe in the future she can be more honest with her partner.