How can i keep my messages from staying in my outbox and not sending?

I have a i465 motorola cell phone for sprint. My messages keep staying in my outbox and wont send! it makes me mad. can anybody tell me if its locked or something. i need help A.S.A.P|||when ever my phone does that it is usually because i have to many messages already in my inbox so if you delete the old messages most likely it will help send the new ones