First, learn about how babies are made. It can be helpful when you're obsessing to do some research on the subject. That way, you'll learn something while also hopefully demystifying these thoughts. You will of course learn what you already know, that this isn't how it works. But that's just the short term. For the long term, if you have a problem of obsessing over things, especially things that are irrational fears, that's a chronic anxiety problem. What you obsess over isn't as important as the fact you're obsessing over anything to the point it's affecting you're ability to function in life. The first step when you have this problem is to go to work on changing how you think. There are relaxation techniques such as meditation that can help, but the most used way to deal with this is to find a psychologist who specializes in anxiety treatment and see if it works for you. If it gets so bad you can't function well at all, medication might prove to be necessary; your psychologist will tell you if you need to do this. There are other ways to talk yourself out of anxiety, but it's hard, so most start with a therapist.
Therapy seems a natural fit for you. You have been told by several people on the many posts you've made about this that you can't get pregnant that way but still ruminate. Therapy is really going to be the only way for you.