What you wrote is quite confusing.  You reference "him," without indicating the subject to which you are referring... a NO priest?  You also indicate that this mysterious "him" professed heresy, and that you are yourself concerned about your own culpability?  I assume you are referring to failing to correct this man when he spoke his heresy... you then state that  you have concern over "being obligated to do it..."  to do what?  Please reformulate your initial statement so we can attempt to steer you in the right direction.  As it stands I cant make heads or tails of your dilemma.
You may not want to hear about the invalidity of the new order priests, claiming to have "read all the arguments" in your young age... but there is certainly objective reason for positive doubt regarding the validity of these new rites, rendering the participation in them illicit.  Further, if you insist on believing they are valid, your obligation not to participate in them increases.  For if they are valid, then they are gravely and outrageously sacrilegious.  Attending them is also objectively dangerous to your young mind and the virtue of faith in your soul, and should be avoided on these grounds.  You may not wish to hear the reasons the new rite should be utterly rejected and avoided, but you are the one who explicitly and repeatedly indicated that you were attending said rite, when (if you assume its validity and liceity) that information has no bearing on your moral quandary.  In Charity, those of us who have more experience and knowledge in these matters are bound to fraternally correct you on these points.
Please dont misunderstand the following l, as my intention is only to help and not to admonish, despite how it may sound.  The last portion of your post shows a serious lack of understanding the basics of morality:  there is no such thing as "a grave sin..." which "isn't mortal..." but "might be just an inch away from being mortal."  There is an infinite chasm between venial sin and mortal sin, the boundaries of which are concrete and well defined; also, "grave sin" is synonymous with "mortal sin."  If you understand a given action to constitute grave matter and you fully consent/will to commit the action, you have sinned gravely/mortally.  If you have insufficient knowledge, partial consent, and/or slight matter; the sin is venial.  A mortal sin is akin to killing Christ and is referred to by St Paul in the verse you mentioned.  Venial sin is in no way akin to killing Christ, and far from rendering the reception of Holy Communion sinful, is actually REMITTED by the reception of Holy Communion.
Of course, this only applies to the true Catholic Sacrament of Holy Eucharist.

This question and several of your other recent posts indicate that you should really pick up a good catechism and read it.  Even listening to an audiobook would help.  You can by no means claim to understand complex issues dealing with the crisis (validity of new sacraments for instance) until you educate yourself on the basics.  You clearly have a great desire to know the Truth, but try not to miss the forest for the trees.  Beware our Lord's words about building upon sand...