has your T given you any alternative coping strategies?
My T's (because I switch often due to financial contraints) have generally tackled the self-harm by dealing with the underlying issues while taking focus off the sh. It seems to work better for me. We also add in other coping strategies as we go. the week before last, T and I worked on visualizing "containment" of emotions and memories. It's not soemthing I am all that good at, so even thoguh I had done it with other T's in the past, we did it again... It's kinda helping... maybe. I'm not sure yet
Other things that have helped have been to be able to journal everything, leave messages for T between sessions (sometimes asking for a call-back, though not often), and trackign the sh urges, reasons, and actions.
The best for me though has been not focusing on the sh. Mine is not at a severely harmful level anymore, so simply noting whether or not it's happening and how often/why is ok. There have been times in the past where this wasn't possible, but at this point it works.
Can you talk to T more about the expectations around stopping, the process T wants to follow, and any way T might be able to help you through this? Sure, distress tolerance is great in theory, but it's ****ing hard has hell in practice. Does deal-making or bargaining work for you? Sometimes I tell myself I need to do X before I allow myself to cut. It'll be something as stupid "I have to color this minion, then I will re-assess my desire/need to cut" and then when that task is up (or time limit), I try another thing to put it off. Sometimes it works, sometimes I end up cutting anyway, but even a few extra minutes can be a huge victory.

Hope you can find a way to tackle it...