Three days ago, I did a terrible thing withto my cat. Three days ago, I did a terrible thing to my cat.

"With": You and your cat did something terrible together "To": You did something terrible

When I did that, I didn’t know that was a bad thing. When I did that, I didn’t know that was a bad thing.

Until today, I just realized how much terrible. Until today, I just realized how terrible.

Why I saidy my cat doesn’t look like before, because before, he was a fun cat, he was so active, he ruan and jumped all day. I say my cat doesn’t look like before, because before, he was a fun cat, he was so active, he ran and jumped all day.

But now, he almost sleepssleeps almost all day, and he doesn’t eat anything. But now, he sleeps almost all day, and he doesn’t eat anything.

"Almost" is modifying "all day" not "sleeps," so it needs to be attached to that.

My sister said it was because I cut his mustache,. I don’idn't believe it, so i searchedI looked it up on Google and now I have to agree with my sister. My sister said it was because I cut his mustache. I didn't believe it, so I looked it up on Google and now I have to agree with my sister.

You imply that you changed, so "now" (or something else to indicate that) is necessary.

Now I'm worried and full of regret. Now I'm worried and full of regret.

Or: "Now I'm worried and regret it."