Why do I feel light headed when wearing a mask and can hardly breathe ?

Because you brain thinks it. There is no reason for this. Those masks do not block oxygen diffusing through. We in the med field wear then for over 12 hours a day with no loss of oxygen. Try a different mask. Sometimes the warmer moister air may feel different to you. You can also take quick mask breaks if that helps you

It impedes the flow of air intake; making your lungs work harder to take in the oxygen from the atmosphere.  A good comparison is a carburetor on a car.  Sure it can breathe better without an air filter; but install the air filter, and it will prevent contaminants from entering the engine block.  

It's same with all, putting mask will protect you from dust particles and pollution however if you walk with mask on your face for long time then you will feel pain in lungs.It isn't good for lungs if you will be walking many hours with mask.

Not a normal problem. I wore masks for several hours a day, in my last job, without anything more than slight discomfort.

Because a mask interferes with good clean air. You don't get as much oxygen while wearing one. But politicians don't give a damn. They just want to control us sheeple. 

It's probably just your imagination.  If it's real, then you have a serious medical problem and need to seek help.  Diane's answer is very good.