How can I adjust my cat to our new dog?

My partner and I got a new puppy about a month ago and my 'princess' cat, Landon, isn't taking it so well. He's acting extremely upset, he mopes all day. Nothing has really changed in the way we treat the cats. They still get fed at the same time, they are still sometimes allowed to sleep in the bedroom, and we still love up on all of them. However, Landon thinks that we don't love him anymore. I really don't know what to do anymore. It's so frustrating to come home to a cat that's trying to drown himself in the sink, having a cat sit on top of my face, etcetera.