I do these loops all the time... I'm a master of them.... It's really crippling actually, my mom does this too, I think I got it from her, and when we're on the phone it's like "The What If? Show" with SNS and her Mom.
But anyway, my best advice to stop them is just to put one foot in front of the other... It's the only thing I found helpful.... Like if I'm getting a job, I'll let the what if cycle go on because it's nearly impossible to stop it for me, but at the same time, I'll just say, "well I'll just write my resume one section at a time" before I know it, all the sections are done, and then the next thing is "well I'll just drop it off." It's the only thing you can do to overcome it, and then as time goes on the loops can quiet down a bit and you feel a little more confident in yourself for being able to overcome your own mind. I've done this for every big situation in my adult life.... Baby steps.
Baby steps to the door.... baby steps down the hall..... baby steps down the street..... baby steps to the place..... baby steps to the counter to ask for an application.....
These loops can DESTROY YOUR LIFE! If you let them... don't let them!!! Take it from me.