Yesterday, I was stressing out over my uincomplete assignments, but, ten years ago, I was stressing out picking outabout choosing my school uniform. Yesterday, I was stressing out over my incomplete assignments, but ten years ago, I was stressing out about choosing my school uniform.

There are many prefixes for "not", we use "in" to modify "complete". I would change "picking out" to choosing since you used "out" in "stressing out" twice already in the sentence. This means the same thing, but avoids the repetition.

I haved nothing to worry about when I was 13 years old. I had nothing to worry about when I was 13 years old.

"Had" for past tense.

I was living my life to the fullest and now I'm surprised by how hectic the life of a 23 years old life can be. I was living my life to the fullest and now I'm surprised by how hectic the life of a 23 year old can be.

I cannot wait to graduate and bagged my TESL degree for real. I cannot wait to graduate and bag my TESL degree for real.

"For real" is slang and would be used casually.