Eren had a lot of trouble facing the training regimen. Like Captain Hitsugaya said, Rukia was really planning on sending him back to hell. The training regimen hurt him like hell on the first day itself, and Soi-Fon just taught him two moves. Of course, he had to do both the moves 1000 times each and that too by a certain time. He had to endure a lot of pain in his arms, legs and torso. Every part in his body seemed loose and it looked like limbs will fall off at any point. Enduring all the pain and the broken joints, it took him nearly an hour to go to his quarters that was only a block away. Rukia took a lot of sadistic pleasure when she passed by as she watched him stutter in his steps with exhaustion after training.
Despite the extremely harsh training, he was thankful for it. It made him so exhausted that it made him sleep faster. Of course, he still had a few tears before he actually went blank, but it was better than staying awake for the whole night. These were how his days went…training day in, day out while the memories in his head threatened to keep him awake at night. There were times when he woke up at night to train…train in order to keep his mind off of the thought of what may be happening in his world…with his friends. Are they fine? Are they alive? Has everyone moved on? Have they been happy with their lives?
No matter where he started thinking, in the end, Mikasa would be the thing on his mind. He would curse himself for not noticing her feelings sooner, At an earlier age. Even if he thought about things that he could change in his life, all of the other outcomes ended with the Eldians being wiped out. And even if he DID get together with Mikasa, there would be no chances of a future with her as he would have died in four years anyway. In the end, this was the only possible way to save his friends and his people.
He usually had various nightmares about the Rumbling and the people who died in it. If not, then he used to have nightmares of his mother being eaten. His most peaceful nightmare had been Mikasa living together happily with Horseface, which brought him to tears every time. But slowly, he used to have void dreams of himself in pitch black darkness, staring at his Attack Titan, which stared back at him. Initially it was just a flash of his dream, but soon it moved to longer durations, until one day, his entire dream involved him staring at his Attack Titan as it seemed to communicate something with him. When he woke up that day, he felt very much lighter. He felt less exhausted and he was able to move his joints properly. As he examined himself in the mirror, he noticed in awe that the cut on his body from yesterday's spar with Unohana slowly disappeared, leaving behind a trail of steam. He then understood what was happening.
Eren smirked
"I see."
Rukia was getting really irritated every day that her plan was not working. She really hoped that her regimen would break him down. But that had the opposite effect. As days became months, and eventually to an year, his skill kept growing. And her training regiment only made him grow at an alarming rate. He was already at the level of an amateur Shinigami ( In terms of Zanjutsu and Hakuda) by the end of his first year in training. Soi -Fon couldn't really understand his body or his pain tolerance. Not matter how many times she ensured to break his bones, he would still come back to training, not missing a single session.
Unohana was soon able to figure out his weird ability to heal, and it really intrigued her. She initially feared it was hollow powers, but hollow healing did not involve steam formation. There was steam formed during Shinigami healing, but wounds didn't heal in an instant like Eren. Although she didn't understood that ability, she cursed it in her head as she was annoyed that it stopped him from giving up.