Actually the case is, deep inside you are feeling guilty. But you are trying to brush it off, and this is what the problem is.
If you really don’t feel guilty, you will not feel guilty about not feeling guilty at all. It is because you are feeling a little guilty, and you are trying to push it away, which is creating a conflict in you. So what you need to do is, simply accept it. Accept the mistake and whatever you need to do for remedial measures, you do it.
Once you accept the mistake and want to do some remedial measures, then you will find peace within yourself. But if you brush off the mistake and act as if it never happened, or you didn't do it, it is then that the prick starts from within.
So the first step is to accept, and then to do some remedial measures. That is it, then it is done.