Last I checked reddit only shows your your last 1000 comments. So it's impossible to get your older comments. I don't know how they do this in light of GDPR's data portability requirements.

See

https://www.reddit.com/r/help/comments/6ynrth/why_does_reddi...

https://www.reddit.com/r/redditdev/comments/30a7ap/does_redd...

But what you can do is download every single comment on reddit, then search through that to find yours. See https://www.reddit.com/r/pushshift/