When I'm feeling frustrated or anxious I try to just write what I'm feeling and work through it. Sometimes that's affirmations (I am good enough, I am safe and loved, etc) and sometimes it's just brainstorming for how to solve something. I find just getting it out of my head and onto paper at least lets me more effectively get a grasp on what's bothering me even if it doesn't fix anything.
Writing things down also helps me realize that sometimes I'm just being unreasonable, haha.
I don't really go back and read most of it, it's more like scratch paper for my brain.
I have tried a few times to journal when I want to let things out of my system.. but haven't found it much helpful in reducing the miserable feeling. Maybe if I am consistent it will change things.