Maybe you could tell her first that you have something important to talk to her about, then ask her to pick a good place that will be comfortable.
Heck, I don't know. Whenever I have anything important to say, I just keep it to myself since I'm married....lol
Seriously, in your case the conversation would probably start off with:
"I don't know how else to start, but I need to tell you how I feel __________."
Give her the best puppy dog eyes you can muster.