Several thoughts:
No, you may not be able stop your 18-year old from drinking and goofing off in college, but you don't have to support that option financially. Though our kids are responsible and recognize that we've saved for their educations -- and though they want to show their appreciation by working hard and honoring the sacrifices we made -- we let them know that they're on the "semester plan". That is, we'll gladly pay for first semester, and when good grades appear, we'll gladly pay for second semester, etc., etc., etc. If we DON'T see good grades and progression towards graduation, it's time to make a new plan. That plan might involve coming home and attending community college, or it might involve stopping studies for the moment. But if our kids were spending their days sleeping in and drinking their evenings away, they might keep doing it -- but it wouldn't be on our dime.
As for choosing majors, that's something with which we're dealing right now -- our youngest doesn't know what she wants to do. She's good at a bunch of things, but not superlatively good at any one thing. She wants to do something that'll help society, but something that'll also allow her to live comfortably. This board kind of leans towards the idea that you should just enter a STEM major -- preferably engineering -- and if that's just not a good choice for you . . . well, what's wrong with you? Are you stupid? That just doesn't work for everyone. I, for example, am super at math, but I specifically remember realizing in second grade, as we were learning to subtract two-digit numbers from two-digit numbers, "I hate this. I really and truly hate this." And being forced to complete worksheet after worksheet after worksheet . . . of the same old, same old, same old . . . even though I'd mastered the skills . . . did nothing to make me like it better.