Start Strong, No Excuses, and Delay the Post-Mortem.

*Start Strong:
Regardless of your skill level, avoid starting a presentation or speech with an apology. 
Don’t begin by telling colleagues that you’ve never done this before. Hold back from asking for their understanding if you make mistakes or stumble. 
Prepare a strong opening and then practice it. You have about 90 seconds to grab your listeners’ attention, don’t squander it.

*No Excuses:
If, during your presentation, something goes wrong, don’t launch into excuses. Stop talking and take a breath. Then communicate that you need a moment to sort out the issue. 

*Delay the Post-Mortem:
Once your presentation is over, don’t immediately dissect your performance. It is still too fresh. 
Live with it for a few hours, then list 2-4 things you did really well. Next, list 2-4 areas for improvement. Consider how you will make those improvements. 
Don’t analyze your performance to the point you become a wreck. It will only make you anxious and affect your future presentations.