no, "sorry, I'm still going" is not used. the other three ways you show are all correct.

You can say all of the above! - "Sorry, I'm still going." --> context clues imply that you aren't physically going anywhere but eating more of your food. - "Sorry, I'm still eating." --> polite and concise! - "Sorry, I'm not done yet." --> same as above - "Sorry, I'm not finished yet." --> same as above If you are at a restaurant you can also say: - "Can you come back to me?" - "Can you give me a few more minutes?" - "I'm still working on it/this." - "I've still got a few more bites left." All of those are fine to say but of course, depending on your context, location and who you are with, certain phrases will be more appropriate than others. :)