I'm in the process of having my book published, and they are asking me to change A LOT. Would you listen to them?

It's just hard for me because I spent years with these characters how they are, and they are asking me to change BIG plot elements. I knew they would ask me to change or cut here and there, but not like this.

I am of course grateful to even have my book published by anyone, but you see my delima. Or maybe not.