Well thats good that your priest is helping you and I hope you continue to go to him as your continue to deal with this issue.
Now the way to distinguish mortal and venial sin is like this, the Catechism says that sins becomes mortal when three conditions are met: 1) grave matter, 2) full knowledge of the evil of the act, and 3) full consent of the will. If one of the three conditions aren’t met then the sin is venial and not mortal. Now more directly for your case I would say to ask the priest who you go to confession to if it was a mortal or venial sin. I’m going to refrain from assuming if it was mortal because 1) I’m not a priest and 2) I haven’t been trained to deal with a sensitive issue like this. I’ve been where your at and I can offer you my help and prayers.
You are correct that all venial sin is forgiven when you receive the Eucharist, but all sin is also forgiven in the sacrament of confession as well. If I was you and I was struggling with if a certain sin was mortal or venial I would go to confession just to be sure. I wouldn’t call going to confession, even if it was just to confess only venial sins “unnecessary” because dealing with sin is a process and using the sacraments of the Eucharist and Penance are far more helpful when used together. You should see confession as an opportunity to go to the doctor for your spiritual well being, and just like any doctor will tell you that you shouldn’t only go to the doctor when your dying with an illness but they encourage going to them frequently. Likewise I feel this is the best attitude with confession.
Now I don’t see that that is true, because if God didn’t listen to anyone who is in a state of mortal sin than there would be no way that a perfect act of contrition could be possible. This is what the Catechism paragraph 1452 says about perfect contrition:
“When it arises from a love by which God is loved above all else, contrition is called “perfect” (contrition of charity). Such contrition remits venial sins; it also obtains forgiveness of mortal sins if it includes the firm resolution to have recourse to sacramental confession as soon as possible.”
So this proves that God would still listen to us, even when we are in a state of mortal sin. When we commit a mortal (or even venial) sin, it includes a turning away from God. God doesn’t turn his back on us, we turn our back on God. He is a loving father who can’t wait for us to ask for forgiveness and is willing to give it unhesitatingly. I would recommend you read the parable of the prodigal son in Luke 15:11-32. This parable I feel shows how God views us as his children.
Hopes this helps and God Bless!