Something that usually works to solve this are to simply clear your cache in the settings under the messaging app. You can do this by going to settings>apps>find the messaging app and then select clear cache. If that still does not work you can clear the phone's cache through a stock recovery. You will not lose any data so don't worry. You can do this by following these steps:
1. Shut down the device.
2. Hold down the volume down and the power button simultaneously until you boot into the HBOOT Menu
3. Select the RECOVERY option using the volume buttons
4. Your phone will show the white HTC screen, then you will see a black screen. It will take a while but it will say "Failed to mount SD" four times. Wait until it turns into a red triangle with an exclamation point inside the triangle and from there, hold down the Volume Up and then press the Power button. You will see a small list, this is stock recovery.
5. Select wipe/cache.
I hope this helps!