It helps to start with the dispassionate and objective question - why are you getting TT? To answer that you need to look at where her TT is coming from, what is her motivation. It's going to be a mix, but parsing it out can help. It helped me.
If a therapist were to ask she might respond that she is sparing you the pain. This one is a complete bullshit answer but it's where you need to start because it's where she is starting. It's almost certainly the "why" she is telling herself. This one needs to be acknowledged and then thrown away.
The real why is one of control - she is controlling the outcome. The term "controlling the outcome" is frequently thrown around on SI, and from the name we've all got a general idea of what it means - she wants an outcome (for you to not say "I'm out"), and she thinks telling you the whole truth is so bad that you will take control and walk. To her mind, your being in control probably means you leaving her, but in any case it's taking the power to decide from her hands and putting it into your hands. Her panic "controlling the outcome" is out of fear, the fear of loosing the stability of her life and her loosing control of what her future looks like. From this perspective the TT is selfish and about her, it's not about you.
Unfortunately TT is damn near universal. No matter how much the BS insists that TT is destructive and they need the truth once and for all, the WS is equally insistent (internally) that it's too much to give to the BS. It's near impossible to convince a WS that it's time to give up the power, to "let go of the outcome", and to give the BS all the truth. Giving the BS all the truth also gives the BS all the power to make decisions for the future of the relationship. For anyone, that power shift would be scary as hell. But now that you are on the last straw, she needs to hear and understand that she's already lost the power to control the outcome. The only way to save things is to come clean, to give you everything, and then maybe with her love and support you will choose the outcome she desires.
Coming clean is also just really hard. To see your face as she tells painful truths is excruciating for her, and she will retreat from causing pain because it pains her. This retreat reinforces her initial "why", the notion of sparing you the pain. This is why a timeline is such a better way to go, have her write it all out and hand it to you. There are many here who can help explain how to do a timeline properly. In my case we didn't do a formal timeline but when the TT was at my breaking point (about 6 weeks out from dday) my wife wrote me a document of critical details that broke the TT logjam and that was instrumental to saving us.