Yeah, I was lucky in that I did not get TT, but I did have this withheld from me for like three years.
It is the lies that kill your chances of R. Give her one more chance to come clean and ask for a poly to confirm what she told you. The rationale is that since she has lied before, the only way you can feel the full truth is by having it verified by another source. Only then will you begin to allow yourself to trust her again.
So why do they do this ? The best answer (other than what others have said) is that some people through various channels (usually FOOs) learn that telling lies is the only way to protect themselves. So for example when you are young and you get asked a questions about something, they learn that lying has fewer consequences that the truth would be. This becomes ingrained in who they grow up to be. Identifying these in IC is the only way to "fix them."
Lastly most of us get confused by the way our WS because we are looking at them from a vantage point of being "unbroken." Looking at a person with unhealthy mechanisms from a "healthier" vantage point does not always work. It is someone who is blind and someone with perfect vision looking at the same thing and trying to describe it.
I am in no way justifying the behavior, but sharing what has helped me deal with the lying issue.
Have you set any boundaries or consequences in the past for TT ?
In the end it will not be the A that ends my M, it will be the lies. Most BS see it that way, after some time. Lying of any kind, is a deal breaker for me. Even about something small, like did you eat the last cookie ?
Lies lead to D. You have to determine if she has the potential to be 100% truthful with you. If she can't, then you have to decide if it you really want your M.
Sometimes you have to be ready to lose your M to save it. A wake up call, like a poly or S agreement can be powerful motivators. They aren't threats, they are just a different language that someone who isn't as healthy speaks so they understand 100%.