There is a way to find your past YouTube comments and there are various ways you can find them, they all need the use of Google to find them.
Enter this in the Google search bar.
/all comments YourUserName
This will give you results of most of the videos you have commented on, if you don't see a video you commented on it's possible that the video may have been deleted.
This method also gives you videos your own videos may be related to.
Enter your actual comment into the search bar. If you can remember what you said(or most of it) and include "site:youtube.com" you will most likely find your comment.
May keep this updated if I find out other methods.