Rest assured, Reiki is a natural energy that adjusts to the needs of the recipient.
Being afraid of healing is definitely a normal response for anyone who has suffered because it is an unknown.
If you have a therapist that you can discuss your fears with, I would continue with that. A therapist can provide you with tools to understand why things come up as fear, when they do, and what you're afraid of.
For the Reiki, start thinking about it like a warm bath or a nap on a hammock, or put your mind into the clouds and let them drift along. Using imagery like this might help put your mind at ease while getting a treatment.
Let me give you an example of how Reiki may help you in an anxiety situation. Reiki, natural and gentle, adjusting to the needs of the receipient, may actually bolster your resilience so that if you have a time of anxiety, you are able to move through it easier. In this last Reiki session you were able to not have a full blown panic attack and come through it with the ability to analyse it and perhaps gain some understanding of self.
Was it the Reiki that caused the panic or was it the Reiki that allowed you to navigate the panic?
If it were me, I would continue with therapy to get tools for my mind to work with and I would continue with Reiki to have energetic/emotional/spiritual support healing.