When I finished The Gastien Series I sobbed like a baby. Ruined my make-up, gasping for air type of sobbing. It was so hard to let them go! I posted in here to ask if it was normal. Within minutes I had a couple pages of responses, showing me my reaction was not unusual.
I was convinced I would never find another character to love, let alone several. Nor would I have a decent idea for one more book, let alone a series. I took about a month to just paint, as that is who I am in my soul.
Then Dave and I were driving down the highway about 6 weeks after the final Gastien came out, and he asked why I hadn't read 50 Shades of Grey. I told him, and then, in my mind the words "House of Pleasure, House of Pain" came clear as day. I looked at him and said, "I know my next series. It will be about some girls, and one boy, who are sex-trafficked. It will show the pain behind what men see as pleasure. And it will be about revenge." He asked what the name would be. I said, "There will be four books: House of Pleasure, House of Pain, House of Trickery, House of Shame."
It was difficult to start and I felt empty as I typed away. I am a total pantster, so the main character takes over and I just type. I kept going, feeling the whole thing sucked (but then I always do while I'm typing it.) I wanted to quit, but knew I needed more books. When the first was done and I read it, I got excited. The characters meant something to me. By the second books they meant as much as the Gastien characters had. Well, almost. Gastien will always be the series with the most of my soul in it. (He, too, was an artist and so was his grandson.)But all the rest I have grown to also care for deeply.
Just start writing once your series is finished. You need to "grieve" the loss of your finished series, but you also need to let those new voices dying to be heard come out. Good luck. You WILL find yourself excited again. I promise.