you certainly can say no any time it gets to be too much. and I would encourage you and T to keep an open dialogue about how you are feeling. have that conversation with her before you guys start (I'm hoping she is already planning that out, but in case she doesn't, you need to bring it up).
just keep in mind that you will need to push yourself past your comfort limits. it will likely suck a lot, and be really scary. if you can trust your T & yourself though, it might prove really beneficial.
I know when I did emdr for some stuff, it was scary and triggering. It helped a lot to trust my T though. We developed a safe place and laid the groundwork for what to do if the emdr became too overwhelming. I panicked and wanted to back out a few times, but she was good at guiding me back through it all. she explained and re-explained what we were doing, why, and how it was supposed to help. she often checked in to make sure I was ok, even if I wasn;t expressing any discomfort.
I'm assuming (hoping) that your T will work on the safety piece first as a precurser to the actual pe...
go in with questions, tell her your concerns and worries, and come up with that safety net together. ask her to explain it in detail to you, what you guys will be doing, how to handle getting overwhelmed, when to push you vs when to back off a bit... it's scary, but ultimately you are still in control of it.
you can do this! <3