What is my phones issue and can you help me figure out how to fix it?

OK So i hav this stupid phone that usually acts normal- i havent dropped it any time soon and its been ok apart from the usual battery decrease from texting too much (but thats my own fault) well i have unlimited text and i have great service and right now it keeps on acting up and every time i try and send a message it will get almost done then say Error Resend? crap and i just want to text this person back. Iv already tried turning it off and then back on and its still acting up. I have a Sony Ericsson, walkman slider.