Since you said that you were so afraid that you were going to cut yourself, could the pressure on the cutter be changing due to you changing it for fear of cutting yourself.
I have poor vision and I was always afraid of cutting myself, so I bought one of those handles with the the suction cups to hold the ruler on the material while my fingers stayed way away from it. It is great and I no longer am nervous when cutting. I paid about $16 for mine but I saw them at Menards the other day for about $6 or 8 dollars.