You are your own worst critic. When you practice alone (especially in front of a mirror,) you will nitpick every single thing that you do wrong. You will be overly critical of yourself. In addition, when you practice alone, you will not get any feedback from an audience about how well you are communicating. A better way to practice is with a friend or coworker, though.
... Wait, wait, wait, a minute... you just said that friends and coworkers are terrible coaches? Now you want us to practice with them?
Yes, the verbal feedback that friends and coworkers give us can have a negative result. However, the VISUAL FEEDBACK that they give us is vital. For instance, when we are communicating with someone and they nod their head as we speak, we are getting a visual cue that we have communicated effectively. When we say something that is confusing, we will see that in the face of the listener as well. These visual queues allow us to make appropriate adjustments to our speech. You won't make these adjustments as frequently if you are practicing alone.
It is kind of like practicing a bad golf swing all alone. They say that "Practice makes perfect". However, that isn't true. "Practice makes permanent." So, if you practice a bad golf swing over and over, you will get really good at a bad golf swing. If you practice a presentation alone, you will likely get really good at delivering a bad presentation.
So, practice once or twice with at least one person. You will then be able to make appropriate adjustments to the presentation to increase better communication.