When I was 18 (and 21, and 25), I was far more likely to put up with bullshit in bad relationships. When I love, I feel intensely loyal, and it is very difficult to make decisions about what I need to do to take care of myself.
But the most important thing I've learned over time is devote more energy and loyalty to myself. That has given me valuable relationship skills, and has helped me develop healthier and stronger relationships with my lovers.
To the OP: Those of us who are significantly older than you have had the experience of living through and learning from some hard knocks, and of course we would like to somehow communicate to you what that's like, and what we've learned. You, of course, are not going to be able to hear that. And honestly, you shouldn't. There's no reason for you to "skip ahead" and try to look at things with the jaded eyes of a 30-something or 40-something or 50-something, etc. They say you shouldn't trust anyone over 30, and there's a reason for that. Be 18. Experience 18.
You describe a mess of a relationship, but this is the time of your life to be experiencing that. I could suggest that you leave this couple in the dust and go out in search of happier, healthier relationships, but you're not going to know what those relationships are like unless you ride this one into the bitter ground.
Meanwhile, though, take notes. Keep a journal. Learn from your mistakes. And try to do something generous for who you will be in 3, 5, 10 years: don't do anything permanent. Avoid the heavy drugs, and the babies, and over-spending on credit cards. Be very cautious about student loans-- you can't wipe those out in a bankruptcy.
As for this relationship, try talking to your partners about your feelings. Use "I feel" statements, rather than "you should" or "you do this" or "you do that." Pay attention to what works in conversation, and what causes arguments. Speaking with a low tone, quiet voice, can sometimes keep the arguments from starting. Don't keep a tally of who has sex with who. Keep a tally, instead, of how many nights you feel alone and sexually frustrated, how many days you feel unhappy with the relationships, as opposed to how many days you feel satisfied. Use that to judge how much more energy to put into this. And good luck.