Urgent Help please. I can't mute my microphone when sharing my screen

Hi,  I have been through countless settings and help sites and can't find the solution.

I am connected to zoom via my computer audio only.

I am using a microphone for my audio in and speakers for audio out.

All is well, until I share my screen.  Then my listeners get a huge echo.  So I mute my microphone and they can still hear me!

I can't seem to mute my microphone whilst sharing a screen.

I definitely don't have anything else picking up my voice connected to Zoom, so why is this happening? 

Big live presentation due in 2hrs, if anyone can help me I will be eternally gratefull.

Thanks