Walking on egg shells with your partner is never fun ...
If you feel saying it to her face that you decided to call off the wedding maybe write a letter to her sometimes it's easer to do that then saying it to there face .

Just be ready for the get out of my house , I'm sure she won't take it lightly , no matter how you say it it's going to hurt her , but it's better to be honest with her now then if you got married .