A question about diaphragmatic breathing Why is it so hard?!

Seriously though, my understanding is that I must simply open my abs, back, sides and ribs in order to encourage air to move into my lungs. 

I am getting better, but I find trying to use this method alone leaves me incredibly short of breath. I'm finding I am 'pulling' some air in as well (which is much easier when the diaphragm is opened). 

Is it ok to do this? With more practice, should I be able to get all the air I need by simply opening up my diaphragm? 