I've thought about the same girl everyday for the past 18 years. She's not my wife.
You have thought about this person for 18 years?
Yes, I should have married her instead
Keep in mind that marriage is a commitment. It's not healthy to keep thinking about some past romance when you are married. It's best to try and bring romance back to your marriage.
I can't do that, my wife is a total bitch.
You've likely romanticized this other person after all this time. You should either commit to your marriage, or end it. It's not fair to your wife for you to spend your time calling her a bitch and thinking about someone else.