How do you get through an artists block?

I like to sketch and paint. But I always have the same issue. I will get inspired by something, currently, it's koi fish and glow in the dark paint, draw it out enthusiastically till it's ready for paint. Then I get stuck with trying to find that same inspiration to start painting. I have some sketches that are years old that I haven't finished. It's really bad when I'm trying something new. Because I'm worried about ruining my hard sketched work by doing a bad painting. How does one get past this? This is a pic of the most recent art I've done. But I'm wanting to fix the tail fin. It's an axolotl and a mermaid.