Consider making a general confession: "A general confession", says St. Francis de Sales, "is for the majority of Christians a necessary means of salvation. It gives us a more complete knowledge of our- selves; it fills us with a wholesome shame at the sight of our sins; it relieves our mind of much anxiety and gives peace to our conscience; it excites in us good resolutions; it shows us God's wonderful patience and mercy towards us; it enables our confessor to direct us more safely; and, finally, it so dilates our heart, that we are ever after able to make our confession with greater confidence."
What you do is sit down and write down all the sins of your life, even those you already confessed. You have to be diligent and honest. Let the priest know you are going to make a general confession. After you have made this, all your sins are forgiven included the ones you forgot to mention. If you remember a sin later on, you do not need to confess it because in a general confession, all your sins are forgiven provided you were honest about trying to remember.