How Do I Cancel a Mail Drop When Cancel Is Not Given As an Option?

I am using Mojave 10.14.3. I was trying to send a 7 min. video to a friend. Got a message cannot send through email because it is too big. Asked if I wanted to use Mail Drop through iCloud. I never used this before. So I tried it. Saw that the blue line on left side was hardly moving so I tried to cancel but could not find a way to cancel. After about an hour or so I got a message saying Unable to send attachment with Mail Drop Try again Later with a big blue OK box at the bottom right corner.
Where is my option to CANCEL?? This infuriates me so much. The option you want is never available. I can’t delete the email or delete the video in the email since it is Frozen with that message pop up. I will never use Mail Drop ever again. The Help does NOT help. It rarely ever does.
And this is supposed to be user friendly??
You have to be a computer wizard to know how to do things without having problems.
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Sue