I think this is something that happens to everyone, not just you, including people who are not non-resident aliens. In fact, it happens to everyone everywhere. So you're not the exception to the rule.

The reason why you feel the way you do is because you've come this far knowing what you're supposed to do, suddenly what you know has been completed and your periodic quarter life epiphanies ask you what next? You don't have the answer, you shouldn't have the answer, most people don't have the answer and they start to panic.

Most people in their 20s don't realize that their 20s will never come backuntil they turn 29. It took me two years and plenty of not-so-good people to really grasp this concept . While I may not be here with my family, I do have a tight circle of friends from all cultures who've stood with me throughout the worst times. I know I'm not gonna have time to enjoy life with them later on as we all settle down, so I take the time to enjoy life with them. I spend time with family when possible, despite the gen gap, because I know they're the only ones who care about my best interests. Understand that having and knowing your purpose in life is a comic book theme, not reality. 

As cheesy as it may sound, so much is under your control than you think. You're in a "shitty relationship" you say, what's keeping you in it? Get yourself out, meet other people despite challenges, you'll learn from it the least.  Develop a hobby, like someone said earlier. This can only be accomplished by doing new things. If you're in college, that's even better because so many different people are around you. People who are working don't have this luxury.

So stop thinking and drinking, get back down to reality and focus on enjoying life. Good luck. :)