At 18, you are an adult in every sense of the word. Sure, your frontal lobe hasn't fully developed, and you're going to make a whole lot of bad choices over the next seven or so years, but the difference between 17 and 18 is that you're going to be solely responsible—and accountable—for those decisions. That means the way you engage your personal relationships and the decisions you make for your future are on you, despite your relative inability to properly rationalize them and your lack of experience in making them.

If you're fortunate and privileged, you'll still have the support of parents, guardians, and other mentors who push you in the right direction. If you're even more fortunate, you get to spend these years in a structured simulation environment called "college," where you get to engage with other frontal-lobe-ly challenged "adults" where you can go on adventures and make all kinds of terrible decisions that you'll look fondly on and/or regret for the rest of your life. But regardless of whether or not you attend this life-simulator, you're going to have to start getting your act together and taking responsibility, which is why the behavior of this 18-year-old, despite her tragic background, needs to desperately be adjusted.

This stepmother has been really struggling to develop a closer relationship with her 18-year-old stepdaughter, who has not adjusted well to her blended family, saying horrible things about her stepmother, who she has known as a parental figure for most of her life. This tragically complex situation is probably a signifier of some underlying trauma that the young woman is dealing with, but it prompted the stepmother to turn to this online community to see whether or not she was in the wrong for telling her stepdaughter to leave their home, to which the young woman complied.

Learning boundaries is important, even if it means being disappointed—not everything is always going to go your way. Sure, when you're one year old, something you were interested in suddenly disappearing might be the worst thing that's ever happened to you, but it's important to learn that lesson now because once you're older, learning that same lesson gets a lot more inappropriate and embarrassing. There's a big difference between a 12-year-old throwing a public tantrum because they didn't get what they wanted and a one-year-old crying for the same reason.

With parenting, there's a delicate balance to be struck between giving kids the best childhood possible and making sure they learn the right lessons. One day, that kid is going to be an adult. Never being disappointed by anything during their development as a kid is going to lead to them becoming a spoilt teen and then a full-grown, entitled person. And at some point, it's going to be too late to set them on the right path without serious self-evaluation. 

As commenters have noted here, the teen is the real parent in this situation… and the stepmother's response hints at a possibly unnecessarily hostile attitude toward her stepdaughter.