Feel like I'm going crazy! ANIEXTY POST, PLEASE HELP!

So I have had awful aniexty all my life. Now I am in a relationship with a man that I am so in love with and know for sure that I will marry. The type of aniexty I have, well one kind. Is scary unwanted thoughts. All people with aniexty have gotten those scary thoughts before. But now, my brain keeps popping things up like "I wish I was with David" or "Being with David... *insert something dumb*" or " I wish you were David" Or a completely dumb thought about someone! David being a guy I was with before. I do not want anything my mind comes up with, I am so in love with the man I'm with now and we are so happy, and I don't want anyone else!! I don't know why I keep having these thoughts and it's pissing me off. I feel as if I'm going crazy. Does anyone else have these thoughts??? It would make me feel so much better if someone told me something! So I don't feel like such an awful girlfriend! Thanks!