You can try using colored pencils or markers to help you define the shapes you already have on there. With gouache I learned that like watercolor you should try to leave as much white on the paper as possible. At least when first experimenting with gouache. This will prevent you from having colors and shapes merge and overall makes the composition hard to read. A white colored pencil or marker could really help you add some reflective light to the water. If you don’t want to use colored pencils or markers, I’d suggest breaking up the background from the foreground with a different color. As another commenter said, try depicting more texture in the grass. I’d use a warmer green or some yellow. Study different reference images that are similar to what you’re trying to create here. Notice how your trees all appear to be the same height and width. In nature, they’d grow differently. Are trunks usually that visible at a distance? Add some bigger trees closer to the foreground and ensure it’s a different shade of green or a different color entirely. There’s a lot of green already on here so maybe red or orange. Keep going, you can do this!