First make sure you have the most updated software on your phone.
From the main screen, touch menu tab
Touch About Phone
Touch System Updates
Then try doing a soft reset on the phone.
Press the Power key.
Touch Power off.
Press the Power key to power on the device.
Then dial *228 send and option 1 on the phone.
If you have unsent messages you should see a bubble with a number on the + icon on the upper right hand corner when in the message window and you can then click the minus marker next to message and it will be deleted.
When you send a message, and the message is not sent, it should still be in the thread of the conversation. If you go back to the thread, you should be able to access the message and then re-send the message, by pressing the message and then trying to send the message again.