Here is a short article written by a Catholic convert about the importance of regular Confession.  By going to Mass and receiving Holy Communion you will maintain grace in your soul but if you are living in mortal sin, you won't.  In fact you may be doing yourself harm. You may be commiting mortal sins without full knowlege e.g. adultery, contraception, pornography, and missing Mass on Sundays but you are still culpable to some degree.

http://catholic.org/hf/faith/story.php?id=46833