My mobiles deleted its own memory!?

Ok i have a nokia 5800 expressmusic ans it has an ok memory without the sd card and it can hold up to around 250 inbox messages, but the past couple of days it has been telling me to delete some stuff as it doesnt have room for new messages, so natuarlly i deleted my entire inbox of 250messages and then my inbox starts filling up again i get to about 50messages and it tells me to delete some more things as it memory is full, so i delete it and now today i have to keep deleting things to recieve a new message and i only have like 19messages in my inbox, i dont have apps on my fone, i dont have music or pictures i just use it for the internet and sms. please help me make my fone regain its memory capacity as i love my phone lol.