The whole thing looks flat — like a sticker, not a painting

FIX IT NOW

1. Create a new layer. Set to Add or Screen. With a very large soft brush at 10–15% opacity, paint a subtle warm glow centered on your main light source area. This separates the lit area from the rest and removes the flat even-lighting that makes things look like stickers. If you don't have a clear light source — pick one now (top-left is the easiest default) and commit to it
2. Create another layer above everything. Set to Multiply. Using a large soft brush at 20–30% opacity, paint a dark vignette around all four edges — darker in the corners, fading toward the center. This adds depth and pulls the eye inward, but it's a complement to the lighting fix above, not a replacement
3. Add the texture/grain layer on top if you haven't already (see T6 "looks digital"). A surface texture is the single biggest thing that removes the sticker quality from digital art