- you need
Power Delete Suite can alter or delete your previous comments and posts and acter that you can remove your account
It can do both in one run, as well as making a backup. However, only about half of my Reddit comments got archived in the CSV (also, you can only download it after the script finishes so I always make sure I can leave it running for several hours, which it needs because of rate limits and hope it doesn't crash – it never did, though).
Afterwards did you search for your username + site:reddit.com ?
I found power delete only did most of the comments. I think can only see the top 1000 of each sort method.
After you do what others suggested, go ahead and file a GDPR request to scrape your data on reddit. That's a legally binding procedure and they have to comply.
Here's a handy guide with a not so encouraging message at the end.