I see a lot of this at https://www.google.com/search?hl=en&as_q=send+text+imessages+on+iphone&as_epq=can+t&as_oq=&as_eq=&as_nlo=&as_nhi=&lr=&cr=&as_qdr=all&as_sitesearch=&as_occt=any&safe=images&tbs=&as_filetype=&as_rights=#as_qdr=all&hl=en&lr=&nfpr=1&q=send+text+imessages+on+iphone+%22can+t%22&safe=images
Given all the reasons for this, I think you need to head to your carrier's support counter or an Apple store.
I have an iphone 4. I can't send text messages in imessage. When I do it comes from my email instead. When I receive them they come to my email. I have gone into settings-messages-send & receive-and it is all fine.