People don't understand what I'm saying?

Do you feel anxious when speaking to others? If it is nerves, maybe you would benefit from taking a few deep breaths now and then. Don't feel nervous if people ask you to repeat things, just laugh it off and make a light hearted joke of it, then you'll feel more relaxed too and this will no doubt help with your speech.

I'm sorry what?

No, on a serious note, I know someone that's in exactly the same boat, but you just gotta try to enunciate more and hopefully even the fact that you mumble will be overridden. She finds it really difficult to raise her voice to a louder pitch but she makes sure to pronounce her letters better to make it a bit clearer and has learnt to talk a tad slower (until she gets superexcited and then she's a lost cause). So don't worry so much, try to consciously pronounce stuff slower and fuller if that makes sense and you'll probably grow out of it.

I have this too. I was born with glue ear and couldn't hear very much until I was about 6. I had some elocution lessons when I was 10 and did Grade 1 at LAMDA but I still get a range of reactions to my voice, even now (I'm 21 by the way). Some people have a lot of difficulty understanding me; mainly my mum but she's deaf in 1 ear. However I've also have many people telling me I speak nicely, even being told I have 'a voice for radio' and 'a pleasant telephone manner'. I do get a lot of people asking me to repeat myself though, and I understand that it does knock your confidence.

I think, like you, I speak too fast at times, and I think I'm speaking loudly or at a normal level but I'm not.

Try speaking slower and more loudly, even if you think you're being really loud, you probably aren't. Also, ask your teachers for some comments about how they find your voice.

Wild Precious, I was expecting a pun, you've let us down! Quit with the nonsense above, lol.

Anyway as for the post maybe you should go to speech therapy? Try airing your views to someone who deals with these problems on a regular basis I am sure they would give you techniques to improve your confidence when speaking.

Just sounds like nerves take over when you talk. Some people will rush to get things out or others won't say anything at all.

I used to be like this, it was definitely a confidence issue for me, I mumbled a lot and my sentences were very jumbled. You just need to relax, take deep breaths as much as possible and think about what you're going to say.

Maybe you mumble? Maybe you aren't talking loud enough? I get this said to me too btw....

I talk incredibly fast and people apparently have problems understanding me

It's not even nerves - it's just that when I think I'm talking slowly, I actually talk really quickly. I have no idea how to alter it either.

Have you ever tried recording yourself speaking? Set your phone to record say, ten minutes of just normal conversation with your family, and then listen back to see how you speak. If you make a conscious effort to speak more slowly and clearly, it will eventually become habit like anything else.