How did you become so stupid and weak? First, you're probably not stupid OR weak, just down on yourself because you're depressed about the breakup.
Second, having read your previous questions, he's a bit of a toxic individual who's been fucking with your head. Look what you said here:
"...someone I liked so much, who said he loved me, was attentive, sweet, and loving in the beginning, and then turned into an avoidant drunk who did things like make out with his best friend in front of me (a dude) and then told me to fuck off. What is wrong with me?"
Come on. Read that over again. There's something wrong with HIM. Alcoholics, BTW, are often great at manipulating, blame-shifting, and making others feel like crap. And they only get worse as long as the drinking continues, often alienating everyone in their life in the long term with their selfish, co-dependency-fueling, drama queen bullshit. You're well rid of him. You probably feel like you're losing your mind because he's been engaging in crazy-making.
Third, you are probably a teensy bit right that there's something "wrong" with you, in that you sound like you have some long-standing issues (say, with self-esteem?) that made you doubt yourself and allow this situation to continue. You need to work on those. I guess the obvious thing is to suggest counseling, but, if not that, then just doing something that feeds your soul and makes you feel stronger and more self-assured.
Stop drinking. Take care of yourself.
STOP contacting him. Go cry on shoulders, hang out with friends, watch movies, exercise, organize your spice rack, or whatever. Did I mention NOT to contact him? Seriously, don't. If he calls, e-mails, or texts, don't answer. At this point, it doesn't sound like there's any benefit to you in maintaining any kind of relationship with this person. It's just undermining you.
Hugs (from some stranger on the internet). I have been in similar situations and I feel for you.