Adult colouring books make me feel slightly uncomfortable. Yes, they're interesting and I'm sure they're relaxing, but why would I want to color someone else's doodles when I can draw mine? The better question is: why would YOU want to color someone else's doodles when you can draw yours?
Oh don't give me crap about not having time or any artistic bone in your body. Maybe not everybody can draw like Michelangelo, but everybody can doodle. Doodling is basically a no-holds-barred, stream-of-consciousness approach to creating your own art. Everything goes - random shapes, squiggly or sharp lines, half-formed concepts - and with a bit of color you can make it look a lot better than a pretty adult coloring book.
Why? Because it's yours. You made it. You expressed yourself with it. If your goal is to ultimately relieve daily stress and anxiety, you can take one step further from an adult coloring book and draw your own stuff. It's not difficult, expensive or time-consuming. It's much the same as keeping a diary except your sketchbook is a visual one! There isn't a right or wrong way to go about it because nobody is judging your work.
If you are in fact being harsh to yourself, you must stop. With enough practice you can teach yourself to be kinder and just let the negative vibes go.
Many of my students come to my watercolor classes professing a complete lack of artistic ability. After the class though, I feel that they've discovered something about themselves; they are always surprised that they can draw and paint. Sure, not everybody follows through it in their private time, but those who do end up creating really amazing things.
I'm not saying you should hiss and burn all your adult coloring books, okay. I acknowledge that they're a quick way to get your hands and your mind busy. I'm simply saying that you can do more with the time and focus you spend on them! Buy a sketchbook and your preferred drawing implement (pencil, pen, or marker to start with) and just draw what you feel like, see, or imagine. Draw your shoes. Create a monochromatic abstract painting describing your mood. Write a few notes on the border.
You'll know what freedom means when you create your own lines, your own art, instead of coloring inside someone else's.
PS Just want to clarify that I'm not saying coloring books are useless. My belief is that if you're interested in it or already doing it, you can take it further by doodling and coloring your own stuff.