First of all, there is no rush. You’re still young and the fact that you’re almost done with school doesn’t mean you have to jump straight into seminary to start your preparation to become a priest. Take some time after school to just think and discern what it is you are meant to do.
I’m 23 and that’s what I’m doing right now - just taking some time to figure out if what I’m meant to do is pursue a degree in Theology or not. I’ve given myself about 6 months starting last week. If I still feel that’s what I should do 6 months from now, then I will do it. You could do something similar, perhaps take a bit longer, and see how things go. Pray the rosary DAILY, go to Confession as soon as possible after you give in to sin, and NEVER GIVE UP. Your first step needs to be to take the time to take care of yourself, and that means dealing and solving the problems you are currently having with your addiction. It’s not the end of the world, you can beat this, I’ve heard of many men (and know one of them) who have beat this addiction.
Also, someone mention this could be a psychological issue…take the time to look into that. It sounds like you’ve made up your mind, but have you made up your heart regarding priesthood? Maybe your current problem with porn and masturbation is something that’s telling you you should be looking in a different direction (as in, not a celibate life - aka something other than priesthood). They’re still problems that need to be dealt with for sure, but I’m just wondering whether, since you’ve already ‘decided’ you want to be a priest, you are not allowing yourself to see signs that you’re being shown?
I may be completely off with that, it is perfectly valid that you’ve known, in your HEART, that this is what you’ve wanted since 6th grade. But I still hold what I said - first take the time to take care of yourself, grow as an individual, grow in your spiritual life…and then go back to discerning whether you’re called to priesthood or not.
I will keep you in my prayers! And know you can beat this! Believe in yourself, stop thinking “I am going to mess up again” and start thinking “I’m strong, Jesus is with me, next time I have that impulse…I’ll beat it.”
God bless and the best of luck to you!