For a sin to be mortal 3 criteria must be met.
grave matter. Turning people away from Catholic Truth would be grave matter. You’re guilty here.
full knowledge. You must have an understanding that what you’re doing is wrong. You were misled to the point where not only did you not know it was wrong, but in fact you thought you were doing good. You’re innocent here.
In fact, you were fulfilling the words of Our Lord in John 16:2 “in fact, the hour is coming when everyone who kills you will think he is offering worship to God.” You were killing Catholics spiritually and thought you were offering worship to God in so doing.
- deliberate consent. You freely chose to do this, but you didn’t know it was wrong. I would argue you didn’t consent.
So you committed a non-mortal grave sin. This can be forgiven with an act of contrition - simply ask Jesus to forgive you with a contrite heart and he will.
In fact, any sin no matter if its mortal, grave but non mortal, or venial, can be forgiven via an act of contrition. The only caveat being if the sin was mortal, you make a commitment to have recourse to the Holy Sacrament of Penance and Reconciliation as soon as possible.