Hey there. Wow, reading your fear about the knives is exactly what a person with OCD would think. You are not crazy. We think the thought, then we obsess over it, then we catastrophize it and then we end up where you were with the fingerprints. All very classic OCD.
My guess, and this is strictly from my own OCD, is that when your OCD flares up, you have some sort of increased stress in your life.
Here is some information about OCD that is worth reading.
http://www.wsps.info/index.php?option=com_content&view=article&id=82:ten-things-you-need-to-know-to-overcome-ocd&catid=0
If you have done this for 10 years without formal training in cognitive behavioral therapy or medication, then that is great for you. You have developed your own coping skills that seem to mostly work until what I think is stress comes into your life and you start the irrational thinking.
If you are tired of all this, then I suggest you seek out the help of a psychologist. They can teach you other techniques that you can use to help stop the thoughts.