Are you shy of real people around you or you feel shy of imaginary people also?
Is this happening at home also?
One way you can tackle this problem is by holding your breath.
You just need to take a deep breath in and then exhale half of it and hold your breath for about 45 seconds or a minute and automatically your stream of urine would start building.
You could also close your eyes while you void.
Consult a psychiatrist for counselling.