Why my phone memory shows that full

First I would update your phones firmware if you haven't done so already. Depending on your carrier, this is usually done by dialing a * (star) followed by a number.

Next I would try to rule out the SD memory card, I would test a different card in it and see if the problem persists.

After that I would try removing and re-installing your battery to force a hard reboot if deleting all messages and media files then updating your firmware doesn't clear up any memory.