How do I get over fear of public speaking?

Hi on Monday I have to speak publicly infront of a large audience. When I get anxious (as I am about public speaking) my breathing is affected. I don't know I over breathe but I find it difficult to talk as if I don't have enough air. I know about relaxing breathingbut i can'tstandon stage doing breathingexerciseswhen i am suppose to be speaking!.Anyone used to public speaking/singing who could help me with any tips please?Thankyou in advance