Always mumbling please help?

Ever since I was a child I've always mumbled. My teeth were really messed up for I had a huge gap between my two upper and bottom front teeth. But when I turned 13 my parents got me braces which helped just a bit. I still mumbled and I'm scared that it's going to be a life long thing. Whenever I get excited, I blurt everything out and it all sounds as if it's mushed together. When I'm nervous my voice becomes shaky and again it's all mushed up mumbling. I don't have braces anymore they helped just a bit but not that much with my speech. I am 17 years old now and want this to stop. Some words I can't pronounce especially if there big ones. It sucks and I'm tired of having to repeat what I say. A lot of my sisters friends who don't know about my mumbling will tease me. So I just tell them I have a speech impediment which I'm not sure in this case is actually true. Is mumbling a speech impediment? What can I do to help this? Thanks for answering!!