I'd work on trying to keep the romance alive with your fiancee. Try to spend some alone time together going out on dates or just making love at home. If you have anybody you can leave the baby with, don't be afraid to take advantage of that.
Being attracted to other people when you're in a long term relationship is fairly normal. Just make sure to keep your distance from your fiancee's friend. Don't spend any alone time with her. Keep reminding yourself of how she's too much of a princess. That way nothing will be likely to happen. Promise yourself not to hide anything from your fiancee. If anything starts to happen, you'll let her know right away. That should scare you out of allowing it to happen.
As long as you continue to give your fiancee the attention she deserves and don't cheat in any way, that's what matters. You can't always help how you feel, and sometimes just trying to stop the feelings makes them that much stronger. But just makes sure it remains as feelings and nothing more.. If you try to remind yourself of her bad qualities and focus more on your fiancee, that can help redirect those feelings to your fiancee as well.
“This planet has - or rather had - a problem, which was this: most of the people living on it were unhappy for pretty much of the time. Many solutions were suggested for this problem, but most of these were largely concerned with the movement of small green pieces of paper, which was odd because on the whole it wasn't the small green pieces of paper that were unhappy.” ― Douglas Adams, The Hitchhiker's Guide to the Galaxy