1. This is kind of long way to find but common people do it like this. Click on the Menu > History > and then select Comments. As you can see in the picture above.
2. This the simple and quickest way to find out. Just flow this link https://www.youtube.com/feed/history/comment_history while you are logged in just type feed/history/comment_history after www.youtube.com/ and there you go you will find all the comments you have ever written.