Traumatic memories are encoded in our neural pathways differently and don't lend themselves easily to words. That's one of the reasons why EMDR is a great tool for trauma therapy - you don't have to speak your trauma to heal from it.

I can't articulate much of my trauma at all, even though some parts of myself (I have DID) can "think" it really clearly. Trying to convert the trauma experience into written or spoken words causes a full on trauma response.

You are not alone.

Edited to add: "Go with that". You don't have to write it or speak it before your first EMDR session, even if that is what your therapist has asked you to do. Your therapist will appreciate you giving it your best effort and stopping if it causes you more trauma. One of the most self-honoring things you could do for yourself right now is to take exactly what you have written above to your therapist, and to let that be enough. (It IS enough). That's right where you are right now, in all your authentic, honest, glory. So...

"Go with that."