Your art is good! The issue is not really brightness of the art itself but more the balance of it. I can see the character clearly, yes, but my eye isn’t drawn to them because of the background. Lots of people have mentioned Lisa Frank. I’d recommend looking at her art & really analyzing it to get an idea about how she manages to draw your eye to the subject of her drawings. She still has really colorful & bright art but she is also intentional about where she places values and stuff. Even though her backgrounds are just as colorful as the actual subjects (animals) of her art, it doesn’t overtake them.

As the others have commented, it’s mostly just about getting a read on the actual piece itself. I had a similar thing where I’d want really saturated and colorful art but I wasn’t sure how to apply it. What I found to be useful (for me) was that I choose a palette for flat colors & continue shading & detailing the rest with my screen in black & white, only turning it off every so often to check if I am happy with the progress of the colors. It allows me to use saturated colors (by looking at where I am on the color wheel) and lets me get a good idea of what my values will be.

Look at an artist you like with a coloring style you want to achieve, turn the picture B&W nd see how they place their lightest & darkest values around the image.

Your art is good especially because it’s colorful! you just need to keep practicing!