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Fix the comma mistakes: There are some missing, extra ones added, and comma-splices.
However I brought it to China and didn't bring it back, because I thought the book must be different so I didn't care about it, the most messed up thing is my mom told me to bring all of my books back and I didn't, because I thought they were heavy, that's it, a sad story.
What is "However, ..... back because .... , so I didn't care about it. The most ...... didn't because .... heavy. That's it. A sad story. (OR That's it, and it's a sad story.)