Working on finding a publisher - check!
What else is there for the aspiring author to do? Plenty.
Few things are as satisfying as finishing off a manuscript. On the other hand, pursuit of publication is long and difficult. Restarting the creative process on the next book gives a sense of progress and accomplishment in that 'in-between' time.
We writer types need that sense of creativity.
Let's face it, when you finished your book, you achieved a tremendous goal, pouring a lot of work into a creative endeavor. You gave yourself a wonderful present, allowing your gifts and talents to be expressed in such a tangible way. And it felt good, didn't it? The truth is, that creative process is almost addictive.
Feed that. Get back to work on the next project. Start making outlines, drawing diagrams, filling out characters, or following whatever system or non-system you use. Study the craft of writing and incorporate that new knowledge into the next story.
Is there a sequel in there in the first book? Or a side story? Ask 'what-if?'.
Personally, every time my book got rejected, I wrote a draft of a new story. Let's just say I have several to work with.
You won't have as much time to write new stuff once your book releases.
It's a good problem to have, but it can still be a problem. If you have a head start, you can keep your readers engaged long after they turn that last page with the promise of the next novel, and not wear yourself out trying to deliver it.
So after that last sentence, ask yourself... was it just for fun? Or was it the first step in a serious, perhaps lifelong, pursuit? If it was serious, take a deep, satisfying breath. Pat yourself on the back. Go for ice cream. Then get back at it.