trilogy ramblings

I am hard at work on book 2 of my new trilogy about two rival princesses who become friends, beginning with TWO PRINCESSES. The WIP is TWO MAGICS, and I am really struggling with knowing how to end this book and where to begin the next one. I have written one trilogy before, unpublished, and I'm not counting the Hound Saga because those books all finish before the next one begins and are related, but are not a trilogy in the LORD OF THE RINGS sense, which is to say that they are really one ...