To find your discussion posts, enter your username in the search field on the discussion board page and hit enter. You will then see a link in a black bar that says, "Show posts by ". Click this, and you will see a list of all the posts you have created or commented on. You can click "View Discussion" in the lower left of these to view the full discussion.
If your comment is in a large thread you can then jump to it using your browser's find function. Press Control and F (or Command F on a Mac) to open the search box. Enter your username again and hit enter to jump to your post.