I think I already answered this in the message to which you replied but I'll try again:
0. I don't know how to prevent the trash can from disappearing from time to time. To restore it:
1. Go to the home screen
2. Double tap the butron in the bottom of your phone to open the task manager.
3. Find the Messages wjndow and icon. Scroll the task manager sideways if necessary. Tap and hold the Messages window in the task manager and slide upwards toward the top of the screen to close it.
4. Tap the first window in the task manager to return to the home screen.
5 Tap the messages app to re-open it.
6. Tap the thread from which you want to selectively delete individual texts.
7. Press and hold one of the texts you want to delete. A dialogue will ask "Copy More"; select More.
8. The text you selected should be flagged for deletion and the trash can should now be restored to the bottom of your screen.
9. Tap the radio buttons in front of any other texts in the thread you want to delete, if any
10. Tap the trash can to delete the selected messages.
Notes:
* This doesn't fix the problem. Apple has to do that with an upgrade. But it does restore functionality with out having to reboot your phone.
* I only lose the trash can occasionally, maybe once a week or a little more often
* Only steps 2-4 above are additions to what one would normally do to selectively delete texts in a given thread.
* To delete a whole thread (a group of texts between you and one or more other people) from the list screen, just swipe to the left and tap Delete or tap Edit, radio button(s), Delete.