There are three conditions to mortal sin: - A grave matter - Full awareness that it is a mortal sin - Committed with deliberate and complete consent

Yes, masturbation is a mortal sin. It is known as a crime against nature, because it is a grave misuse of your sexual capacities (probably not the right term, I think there’s a proper way to describe it). If you’re aware that it is a mortal sin, or a sin in general, which you are now, then that fulfills the second condition. The last is with regard to willpower. If you choose to do it, then yes, that fulfills the last condition. I’m not aware of how much habits play into it, but I’m pretty sure there’s an expectation that you will actively work against these temptations, so simply choosing to not do so is also complete consent. Not sure about the last part, so I hope someone can correct me.