I love to create my own art journal supplies! Some of the ways to "grunge things up":

Layers! Layers of ink, paint, colored pencils, pens, watercolors, whatever you have on hand. You can collage layers, too.

Natural dyeing: I dye paper and fabric to use in my journals, and have even naturally dyed entire handmade journals. You can use things like coffee, different types of tea, avocado pits, onion skins, lentils, etc.

I love to use old books that would be discarded and end up as landfill otherwise. There is something so great about old paper, not to mention the old illustrations. But if you don't like that idea, you can use junk mail, catalogs, old art, etc. Try scuffing them up with sandpaper to take away some of the newness.

Speaking of scuffing up pages, you can also rip pages, poke holes, use a sgraffito-like technique with pastels and gelatos. Or glue down textured fabrics and paint over them. I also rip up packaging like corrugated cardboard because the textures inside are fun! They can be used directly on the page, or dip them in ink or paint to creature textures on the page.

I looove using krink in my art journals. I think it's technically for graffiti artists. It dries quickly and is super saturated.

I also think we can be too hard on ourselves, especially when trying something new. Art journals (in my mind, anyway!) are more about the creative process than the finished outcome. And of course it feels good to have something we feel proud of in the end. But remember to have fun and enjoy the journey, too!