I am 16 years old and was confirmed recently. Cutting to the chase, masturbation has been an issue for me and I have always been sorry and ashamed afterwards. Not too long ago I spoke to a priest and confessed this to me. I was very worried about whether I am going to hell or not and he assured me while it is a grave matter it wasn’t a mortal sin in my circumstances because apparently full consent is not always the case in sexual matters. The issue at hand (which is embarrassing for me to say) is that I have kept on falling into temptation and try my best to resist. I seem to fail and each and every time I feel like a horrible person. I have a good relationship with God and in no way do I want to lose that, I have performed perfect contrition in order to avoid sacrilege as before I did not even know that it was sacrilege to receive communion after a “mortal sin” I am not sure of committing. Can somebody please clarify this notion of mortal sin for me because I truly and honestly cherish my relationship with God and do not want to lose it.

Thank you, God Bless.
Perfect contrition involves no longer being attached to the sin. Any person who had perfect contrition would never commit that sin again. This is why the sacrament of confession is SO important.
To simplify everything, the criteria for committing a mortal sin is that its 1. a grave offense, 2. you’re aware its a grave offense at the time you committed it and 3. You deliberately committed the sin. Now sometimes 2 and 3 are unclear to us. Is it my pride that doesn’t want me to allow to admit to myself that I did deliberatley do this? Am I being too hard on myself?
To simplify the matter for yourself, if you are conscious of having committed any grave sin. An exception can be made if you’re absolutely sure that you weren’t at fault for committing the sin because you didn’t know better or because you really did put all your effort to resist, still committed it and can’t imagine what more you could have done. If its gray area, just avoid communion and go to confession ASAP. In fact, my advise to anyone is to go to confession weekly, bi-weekly or at the longer interval inbetween once a month. Do not look at confession as “Ok this is what I do to get back on track when I’m going to go to Hell and I need to worry and fret that while I’m driving to the Church I might get into a car accident and die before I’m able to reach the confessional.” Look to it as you relying on God’s grace to slowly transform your heart and to cause a deeper conversion. It is you resting on the mercy of God rather than frustrating yourself and beating yourself up with spiritual pride because your pride keeps lying to you that its only good enough if you do it yourself.
Overall stop expecting so much of your own will power. You are a sinner. You are completely dependant on God’s mercy. The only one who ever holds you back from Heaven is yourself. If you’re worrying and fretting over “Oh my gosh I keep doing this. God’s going to send me to Hell. I’m a failure” your main sin holding you back from Heaven is not masturbation or whatever other sin you might be fretting over. Your main problem is pride.
God’s not asking for you to become sinless through the strength of your own will. He’s not telling you to stop committing just the mortal sins, but the venial ones He’ll let slide through some Purgation. What God asks is for us to be humble enough to accept His mercy. Pride is the primary sin we all have to overcome more than any other sin. In fact, pride is the root of all sins of every category.
So when you see yourself sinning in a serious way, realize that that sin is an opportunity for you to grow in humility and to come to the confessional and ask for forgiveness and grace. You will gain more and grow more spiritually by fighting all your impulses that are looking for ways to get out of having to go to confession. Where sin abounds, grace abounds all the more. Trust God and His love and mercy.