Hey there, TrickyCatholic. Trust me, I know where you’re coming from (18 year old male in college. It ain’t easy, brotha)
First of all, Confession cleanses you of ALL sins, so long as you are honestly sorry and confess to the best of your knowledge (You sound sorry to me, so get to confession in the very near future. Preferably this Saturday, and then attend Mass the next day, as it is Divine Mercy Sunday, a day meant for redemption!)
Second, if when you masturbated, you did not know it is a mortal sin, you are not guilty of mortal sin. For something to be mortal, the following must take place:
- It must be a grave matter.
- It must be done with full knowledge of its severity.
- It must be done with complete willingness.
According to 2, you were not guilty of mortal sin at that time. Now, however, if you masturbate with the full knowledge it is mortal, then yes, that is mortal sin.
HOWEVER
Since the sin of masturbation is so difficult to fight against, the Church has stated there are extenuating circumstances that can reduce the culpability of the sin to being venial. These circumstances are affective immaturity, youth (Someone undergoing puberty), habit, and anxiety, plus more (social/environmental pressures). Now, you cannot decide that you fall under these categories. A priest judges that for you. However, you are to tell him that you are young and this is a habit. I highly suspect this is a venial sin for you.
However, do not think that just because it is a venial sin, you can masturbate as much as you want. You are called to fight against it. However, if you fall to temptation after struggling with it, you are still in a state of grace if the priest says it is a venial sin in your circumstances.