Personally, I keep these phrases (among many others) in a note on my phone:

"It's okay to be who I am"

"It's okay to feel this way"

"I'm not responsible for how others feel"

"It's okay to let this go" (this could be an argument, a feeling, a memory, a mistake, a flaw, etc)

"It's okay to make mistakes/it's okay to mess up"

"I'm special in my own intricate ways"

"Breathe in the new, exhale the old"

"I will move forward through this"

"I will grow from this"

Generally, my stress improves when I give myself permission to be who I am and feel what I feel. As far as self improvement, that's always been something I strive for so I don't see myself ever being fully satisfied. At the same time though, it's satisfying to reflect on my past accomplishments and know that I'm capable of improving even more.