No, you're not. You're just unhappy in the relationship, and when that happens it's VERY easy to think you have feelings for someone else if they are kind to you, or flirt with you, or just notice you in some way.
The good thing is, and a point you should make to dh when he comes back, is that writing it down made you realise you didn't have feelings for him, it got rid of the fantasy. And most importantly that this whole fiasco seems to have made you realise that you perhaps don't want your marriage to end???