“The Church teaches that sins which are not grave are venial sins.”
Of course there are venial sins. Many of those are “things that I have failed to do”... For a sin to be a mortal sin one has to know that it’s a sin and consciously do it anyway.
Please point out, with regard to the ten commandants, the ones you do not know to be sinful... And which ones you think are trivial.
I would refer you to http://www.saintaquinas.com/mortal_sin.html