That same site says that oral sex, anal sex, and group sex is alright, which I knew to be wrong even before I found out masturbation was wrong.
That site is not only misinformed, it is a false guide, stay away from it. It will lead you to doing worse things than just masturbation.
Now, what you need to do is talk to your priest. Don’t be embarassed, basically every Catholic guy struggles with this sin. You likely fall under the extenuating circumstances, which could make masturbation, in your case, a venial sin due to youth, habit, hormones, etc., but you can’t make that decision, only a priest can.
This does not mean that you should keep doing it if your priest tells you that you have extenuating circumstances. You should try your best to avoid it.
However, we all fall occasionally. We are sinners, it is impossible for us to completely avoid sinning. That’s why we show true remorse for our sins, and attend confession.
Since it would be a venial sin for you (assuming your priest says it is for you) , it would not prevent you from entering God’s grace if you were to die somehow after committing that sin and before receiving Confession.
Trust me, as an 18 year old guy in college, I struggle almost constantly with sins of lust. Sexual desire is something good that God gave us, but we abuse it. Don’t get the idea that sex is bad. Sex is a wonderful thing when a married couple perform it together and allow for the possibility of conception. It’s also not evil when you find a girl to be attractive or anything, as long as you don’t dwell on it in a sexual nature. Of course I find a lot of girls my age to be very attractive, and I am interested in starting a romantic (non-sexual) relationship with one, and do I fall sometimes and lust for one? Yes. But, I try to keep focusing on the overall value of these beautiful women. They are more than just sexual objects, they have their own opinions, their own interests, and their own hobbies. They are unique, blessed individuals.
You’re not alone. You can fight it. Have faith and confidence in Christ.
EDIT: Also, it’s pretty easy to tell a site is not morally correct when they have a page titled “Fisting and God’s Will”