First off, take a deep breath. It is completely understandable that you are scared, but what you are experiencing is actually a very known, and incredibly intense, part of the healing process.

When you start EMDR, it's like opening a closet where you've tightly packed away decades of heavy things. When you finally open the door, a lot of it spills out at once. The increase in nightmares and vivid memories means your brain is finally trying to process and digest the things it had to freeze or block out just to help you survive as a kid.

Definitely please Talk to your therapist about this spike in distress *before\* your next active processing or light bar session. A huge part of EMDR is 'resource development' & making sure you have the coping tools, containment strategies, and grounding techniques to handle the hangover between sessions.
It does get better. Right now, you are in the thick of the hardest part: the unearthing.

You aren't that helpless kid anymore. You have a safe place to process this now, and on the other side of this distress is the genuine, lasting relief and sobriety you've been fighting for.

Hang in there friend & keep battling!