I know this is an old thread to bump up suddenly, however it's one of the first to come up when you google for this problem so figured it's worthwhile sharing a free solution to this that I recently created.
I've been facing the problem where I couldn't find any free service that worked right, and let me process the My Clippings.txt properly to get all my quotes in a nicely formatted table or into Excel so that I can digest what were the main things I found compelling from a book. I also think that other services don't process the highlights you make on documents that you send to your Kindle too which I found a huge bummer.
Below is a link to the free tool I made which helps you convert your My Clippings!
https://kindle.brendantrinh.com/