Sometimes I just leave a comment on a question, (not an answer) and forget the question again.
but now I want to revisit the question. But could not find it.
How can I find the questions I have commented on?
On your own profile under activity is a list of your recent comments: https://math.stackexchange.com/users/88985/willemien?tab=activity&sort=comments
Go to your profile page and then click on activity and select comments.
In your case you get to this page.
Another way to access your activity page is to hover with your mouse above your name at the top of the page. Then you will be shown a window containing, among other things, a link to your activity page. (Of course, if you do this on this page, it will show your activity on meta.)