Shalom, Yes, saying Kriat Shma al Hamita with Vidui (confession) and Tachanun, helps fix the sin of Pgam Habrit but is not the only thing that will fix that sin. According to the AriZal it is a very powerful Tiukn for Pgam Habrit. Learning Mishna also helps to fix Pgam Habrit. I do not know about 18 chapters in particular, but learning Mishna everyday will help a lot to fix this sin. And of course there are many other things one can do to fix this sin and most important not to do it ever again Hatzlacha R. Anava