I know what you mean by "cancel". You want to stop a delayed email from actually being transmitted while it is still sitting in the Outbox. This is not to be confused with recalling an email- which is something else entirely.
"Outbox" = pending outbound messages (either due to delayed send, or the fact it is simply still in the process of sending.) You usually only see an email in here for any length of time if large attachments slow it down slightly- then it moves to the Sent folder (unless using delayed send, when it becomes normal- as in this case).
"Sent Items" = delivered outbound messages. It's gone. The only option here would attempting a recall.
The "Sent" folder only tells you want has already been sent. So, if an email you sent isn't there, then it will still be in "outbox" and is likely still within your 2 minute delay window.
How can you make it easier to find these emails to cancel them?
Consider adding the "Outbox" folder to your "Favorites" list at the top, so you can always find it quickly if you suddenly change your mind about an email you "sent":