I'm just scared one day he may look at my right hand and forget that the scar was caused by me punching a glass window and that he may forget and think it something else.
He does forget sins
When someone accidentally does something to harm you, they will ask your forgiveness and be concerned they caused you pain. Typically we tell them: "It was just an accident, forget about it." Of course it's impossible to completely forget it. It happened, it's in your mind. But when we say "forget about it" we're saying you're forgiven, I'm not holding it against you.
This is what God is saying. He forgives and forgets. He's not storing it up to come back and you later and say I've never forgotten what you did so you got yours coming.
The Mark of the Beast is something entirely different. It is a willful turning away from God and rejecting God completely. You don't have anything to worry about with this. If this is the source of your distress then I urge you to let your heart and mind be at ease.
Check out this link: http://www.gotquestions.org/mark-beast.html