You need to make a certain number of posts before you have the ability to edit your messages. You have now passed this threshold and you should be able to edit your messages via the "Edit" button that appears below your messages.
Unfortunately, this is a system that we had to install due to spammers, who made posts and then go back weeks or even months later and edit spam links into their comments. This is also why I won't be sharing the exact number of posts you have to make, as we are frequently targeted by sophisticated spam attackers who may read this in the future.
I am sorry for the inconvenience. If anyone else encounters this problem in the future, please just report your post or contact a moderator and we can make the necessary edits for you.