The X-Men are notorious for convoluted backstories, and when writers try to give readers an easy jumping-on point, it tends to make the overarching storyline even more complicated. To wit, when the X-Men were relaunched in 2012 under the All-New, All-Different title via Marvel NOW! branding, it entailed a deep dive back into the earliest days of X-continuity.
With the X-Men at odds, the team divided, Xavier dead, and Cyclops considered an international terrorist, Hank McCoy, otherwise known as The King Of Bad Ideas, reached into the past and pulled the five original X-Men in their teenage forms into the present. This obviously traumatized them all but was especially damaging to Cyclops, who had to come face-to-face with the fact that he would later kill his father figure, and Jean, who was, at the time, deceased. While the time-displaced X-Men was a lot of fun, Jean's story was specifically poignant as it showed readers what could have been if only she'd been a bit more prepared for what was to come.
When the time-displaced X-Men arrived in 2012, it was because Hank McCoy had watched his lifelong friend Scott Summers turn rogue and murder Xavier as part of the events in the Avengers Vs. X-Men series. Of course, Xavier got better, but at the time, this was a devastating turn of events. Hank was at a loss and felt that if they could only go back in time, he could convince young Scott to follow a different path and shock his modern-day self into realizing the horror of what he had done. Spoiler alert: this is not what happened.
While Beast did succeed in bringing the teen X-Men to the present, he did not know how to send them back, and their presence did nothing to sway Cyclops from his then-current path. Rather than simplifying their lives, Hank had only added five vastly under-prepared teen versions of the O5 to a world that had become exponentially more dangerous for mutants. Not knowing what else to do with them, the X-Men began training them.
Of course, while this was traumatic for them all, there are a few things that made Jean's story unique. To begin with, Jean Grey is well-known for being badly written throughout the Silver Age. We found out much later that Xavier put psychic blocks in her mind to keep her from losing control of her powers. Without this protective barrier, Jean's powers kick into high gear, and she struggles. She is overwhelmed by the "sound" of other people's thoughts, and her telekinesis becomes unpredictable and difficult to control.
Besides that, while the other five original X-Men could see who they would later become, Jean's older self had died tragically — more than once. She could see no future for herself whatsoever beyond the life-consuming pain that the Phoenix Force had brought her, and that wasn't easy for her to understand or deal with. In her first appearances in our time, she was erratic, temperamental, and out-of-control.
Kate Pryde, now a professor at the Jean Grey School, had first met the older Jean at the beginning of the Dark Phoenix Saga. This was when she herself was just a teenager who had been kidnapped, and Jean had held and comforted her after saving her. When Kate had stopped crying and been passed over to the other X-Men for safety, Jean had gone to fight her kidnapper. With a devastated teen version of Jean in the present, Kate was able to finally play a similar role for her. When Jean began to lash out, Kate took her aside and talked her down, telling her in no uncertain terms that part of being Jean Grey meant that she had to stay in control, always. Any slip-up would lead to catastrophe. She was kind but firm, and actually took the time to level with her in a way that Xavier had not. Jean worked harder on controlling her powers after that with the help of teachers like Kate and even her future sometimes-nemesis Emma Frost.
Jean gained surprising fluency in her powers. In The Trial of Jean Grey, the Shi'ar kidnap her to put her on trial for her future self's crimes. The Shi'ar show Jean her elder self's life, including the deaths of her family members, in a devastating psychic attack. She reels from this, but she maintains that this is not her and that she never did any of the things she's being accused of. Rather than taking the allegations sitting down, she fights. She learns to absorb psionic energy, which adds to her control over her telekinesis. While adult Jean's telekinesis appears as crackly and electric, Jean's new powers manifested as smooth, circular, and vibrant. When she fights back against the Imperial Guard that once mopped the floor with her older self and the rest of the X-Men, she smiles and says, "Has any of this happened before? Don't tell me. I can read your thoughts. Guess the X-Men are making new history."
In the Jean Grey solo series, Jean came face to face with the Phoenix Force. When her fellow X-Men didn't believe her and dismissed her visions as simply being nightmares of the future, she took it upon herself to visit with several immensely powerful characters like the Scarlet Witch, Doctor Strange, and Namor. As the ghost of the older Jean Grey prodded her to fight the Phoenix force from the psychic plane, she even reached out to Emma Frost for help. In the end, she faced down the Phoenix, and the elder Jean Grey was eventually returned to life.
The time-displaced X-Men eventually had their memories erased and were returned to their original era, an inordinately cruel conclusion to their story when they'd all grown by leaps and bounds in the modern era. Iceman would go back to spend years of his life in the closet, Beast would never learn from his mistakes, Angel would be doomed to become Archangel, Cyclops would still kill Xavier, and Jean would still die. The story felt like nothing short of a tragedy as we saw them get so close to catharsis with the tragic events of their lives only for it to be brushed away. Still, that isn't so unlike many of the best X-Men stories, which almost always involve people out of their depth knowing the odds and struggling against them to their last breath regardless.
For Jean, we saw in real-time what her life could have been if only she had some warning, some preparation, some sense of control over her own destiny. This Jean learned to control her powers in ways that we never saw the adult Jean achieve. This Jean faced down the Phoenix Force and lived to tell the tale. Erasing her journey from history was not an ideal way to conclude her story, but in the Jean Grey series, she noted that her path had intrinsically diverged from her older self's, saying, "Her shadow will always stretch further than mine, but maybe that's cool. Maybe I can learn from her mistakes. Maybe I can be the happy Jean Grey." In spite of everything, maybe she was.