It was either slowly lose cognitive and physical function while ML's mom focused on caring for her dying partner, going through surgery but still requiring constant medical assistance (possibly for life), or dying right after surgery due to complications. She must've noticed something was up, but she never fully knew and she probably didn't even know she was pregnant either before he left.
So both of them lost out on a life together because he didn't want her to be burdened with taking care of him until his death, making it harder for her to accomplish her goals. However, in the end he still lost time and connection because he lost his memories that weren't brought back until he saw the drama. They could never see each other again, and ironically the one who died was not the one with the terminal illness.
Perhaps it would have been better for him to stay, but like he said, he did what he thought was best and all he wants is to see her one last time.
This is something I wrote early on about the series, and I just hope the author doesn't make Hyesung the bad guy and force him to reconcile. If there's no actual discussion and resolution, it's just going to make the rest of the story hard to read. I already dislike the story direction with the cousin and the other girl (I don't like the pairing at all), having such a major plot point fumbled will bring down everything else.
The staying in contact with Hyesung's mom problem. Like I get she escaped an abusive relationship but she had years to contact her son. By the start of the story Hyesung is an adult, so it's not like she HAD to have the father's permission to see him after a while. Also, hypothetically, if she had the opportunity to actually see her son earlier, would she have still kept him a secret? She didn't tell her new family about him even after trying to meet him, so would she have still kept him a secret from her family even if she could see him as a child? I don't like the mom, and I didn't feel sad that her new family left her, like she abandoned her son once, would she also dip out on them if the tides turned? So that whole bit just left a sour taste in my mouth, and unless it becomes an actual storyline with Hyesung being angry at his spouse and child, it'll just be a shitty thing to force someone to accept. What're they gonna do next, secretly take the new baby to see the mom when Hyesung cut her off for valid reasons? For Hyesung to reconcile with a woman who abandoned him and only showed back up when it was convenient for her?