I'm having trouble with my Samsung Strive...?

Whenever i'm trying to texting my friends, it says "Once your message box is full, you will not be able to send or receive messages until you delete some messages. Clean up now?" And I'm not stupid. I know how to delete conversations and stuff like that because i've had this phone for over a year now. It's just that when i look at all my messages there are about three messages in my whole inbox, but it still says that it's almost full. I have a memory card that has 2GB on it and I haven't even used 1GB yet. My phone has plenty of space on it, too. This has happened to me before, so my mom and I took it to the store and the lady said that she hasn't seen it before and that it didnt have any water damage... and because of our warranty thingy, I got a new one. It was fine for a few months, but now it's acting up again. I took the battery, my SIM card, and my memory card out and blew into the holes a couple of times incase anything was stuck in there. So i cleaned it all out and it is still like this. So I want to know if any of you have had this problem and if you have a certain way to solve it. Thanks :)