Unsent sms messages

I am not clear what do you mean by Unsent message.

It would be better if you can please re-post with a screenshot, in order to understand the phenomenon better. Images/pictures speak louder than words. 

Please understand this is a public forum: While attaching a Screenshot, do not provide any personal credentials, such as Card detail, email id, apple id, IMEI number, Serial Number, Phone number, Order ID, if you are sharing a map ensure your location is not identifiable, etc.

Position the text insertion point at the end of the message and tap delete until it's gone.

Here's a trick:

Press and hold on the spacebar on the keyboard and then the entire keyboard becomes a "trackpad".

Best of luck.