I understand your fear. Remembering what happened to you feels as if you are reliving parts of your trauma. But your therapist is right, this is a good sign. It means you feel safe enough now to allow the memories to come back through flashbacks.
It is important to remember that these memories are just echoes of your assault. You are never going to go through the assault again. You already survived it, and what you feel is just the thoughts and feelings from the assault. They cannot hurt you. The assault did but you lived through that and are a survivor. You are safe now.
You are no longer stuck in the past. You are now moving forward. This is probably why you feel fear and apprehension. But this is a very good thing. Anticipation of the next feeling or flashback is frightening. But you can do some things to help yourself.
Write down comforting thoughts. Things like "I am ok" "People care about me" "I have done this before and have been fine"
List people you can call when you experience a flashback. Support can make a big difference.
Think of things you can do in response when you feel bad. A bubble bath, a drive around the block, cup of tea, whatever makes you feel better.
Keep a diary. Write out your feelings, progress and things that help you. This can serve to not only see how far you have come, but as a distraction when you feel bad.
Whatever you do to help yourself, don't try to prevent your flashbacks from coming. Accepting the flashbacks can help them go smoother and help your treatment progress. I know this is hard. But you are well on your way to feeling better. You are strong and resilient. These flashbacks feel bad but you are working through them. And to have flashbacks is very common with PTSD. It is part of recovery. So what you are experiencing is very normal.