It seems a little difficult, juggling multiple projects. Different characters, different tones, different settings. But multitasking is what professional writers do. They're editing one book, promoting another, and drafting a third. When you're working with limited writing time or tight deadlines, the only way to get stuff done is to multitask. In 1992, Steven Spielberg was in post-production on Jurassic Park while he was filming Schindler's List. Nazis during the day; dinosaurs at night. Two movies that couldn't be any more dissimilar. But he multitasked and got it done. He did the same thing years later with War of the Worlds and Munich. In 1999, Robert Zemeckis was filming Cast Away starring Tom Hanks. He filmed all the before/after island scenes first, then took a long hiatus to allow Hanks to emaciate himself. Instead of spending that downtime lounging on his couch watching reruns of Wings, he filmed an entirely separate movie: supernatural thriller What Lies Beneath with Harrison Ford and Michelle Pfeiffer. Many TV actors also make good use of their summer hiatuses by squeezing in movie roles. Only a pro like Melissa McCarthy could go from Mike & Molly to Bridesmaids.
TV writers staffed on multiple series have to juggle different characters and plotlines. Chuck Lorre produces both Two and a Half Men and The Big Bang Theory. While tonally both shows are the same (laugh track comedies), the characters and comedic rhythms are not. Ryan Murphy wrote American Horror Story and Glee concurrently -- two shows nobody would ever group together.
It's ok to succumb to the Shiny New Idea. We're human. We crave new things. A Shiny New Idea can energize us, inspire us to jump back into writing. But it shouldn't take up all of our brainpower. We have to learn to juggle projects new and old. It's all about multitasking. On the bright side, if you can manage this, then you'll never be bored.