There is nothing from an anxiety perspective that you cannot do. “Can’t” is a choice when we are talking about OCD.
If you say that you can do exposures to the floor, doorknobs, and shoes, but you can’t touch a garbage can, you have just made a false statement, you can touch a garbage can, but you are choosing not to.
If it were true that you couldn’t touch a garbage can, the way the physical universe is viewed would have to be changed; you would be the only person in history who could touch anything but garbage cans. When we are talking about anxiety, there is no such thing as can’t, there is only will you do it or won’t you.
Plus, if you skip doing something in therapy, that means you are choosing to fear that thing, and that just does not make sense to do. Why would someone choose to face all of his or her fears but one? Skipping something is not a good option.
Working with a therapist to break the exposure down into manageable parts would be the best solution for you to face anything you think you cannot.