"I think."
"I believe."
"Double space looks better."
"Single space increases readability."
Blah blah.
...opinions as if they're fact...

Check out http://en.wikipedia.org/wiki/Double_spacing_at_the_end_of_sentences for the only actual studies done on this topic. Because "it's the way I was taught" is an absolutely horrible reason to say it's better. That might be the reason why you find it hard to switch...but you should avoid saying it's better because you were "taught" that way.