OP, I was in a similar situation not that long ago. I chose a mountain bike to commute to and from work. And yeah, I heard about how it's majorly unsuited for my purpose.

A lot of the people here do have very good information, which is why there are some suggestions for a different style bike. But people are right, you have to have fun if you are going to stick with cycling. You have to want to be on your bike. Sometimes emotions are stronger than engineering the physics of what is effective. Have fun.

And don't mind the arguing. I still feel really new, but it seems that bicycles have their own version of Mommy Wars. No matter what you do someone knows of a better way of doing things and what you are doing is all wrong, but at the same time what you are doing is right and the better way is all wrong. Mirrors vs no mirrors is debated with the passion of daycare vs stay-at-home.

When you settle on a bike, take pictures.