This takes a little will power and practicing but it works, I know, I have NEVER had nightmares since I learned to control it.
The thing you need to do is, change what's happening in the way you want, if you see what I mean... If you are trapped in a burning house, just jump and say to yourself I CAN FLY, I CAN FLY. then you should get wings as a bird or a broom like Harry Potter or whatever.
I think that if it was me who had those Rachel-dreams I would think this way.
1. The Visser 3 thing is a very good start
2. You said the other animorphs was nice to you.
One thing I would try is this. When Rachel is bullying you, just tell the others that this Rachel is Visser 3, then she will have everyone against her. They HAVE to stop her (The first time this can be very hard so you have to SCREAM IT AS LOUD AS YOU CAN)
And when you can do that, make her be Visser 3. (all you need to do is thinking: Rachel-morph-Visser 3 over and over again until she does.) Then you got an adventure, you have to find the real Rachel. Then you have a nightmare that you really don't want to wake up from, before you know how it ends.
But keep in mind that this isn't anything you will learn in a week or two, it's hard work and practicing. But you have my word on that it works
Welcome, by the way.