Also heads up I dont think he told her that he loved her cause she was under the impression he didnt. Her trauma doesnt disappear because oh he secretely love her. He isnulted her after childbirth, he insulted his child. He absolutley humiliated her and you think thats ok because he loved her and it was part of a plan. How hard would it have been to let her know that 'oh yeah elena im going to riun your selfesteem cause you months of trauma but dont worry i still love you'.
Well we see in chapter 39 and the second or first chapter that he insults her quite a lot. Its also mentioned in the comments that he had to neglect her and her child. So yeah also after child birth he says that this was a mistake. He means that he shouldnt have been so mean or something, but he doesnt clarify himself and he really ends up hurting the FL.
People really keep saying they cant hate the crown prince until they get the bigger picture. Even if he turned out to be a great person later on or to other people that doesnt negate the effect his actions had on elena at that time.
Spoilers....
Him loving elena secretly or smth doesnt cancel out the fact that he really hurt her emotionally. Especially as her trauma is probabaly going to go on unresolved because this version of him can never really apologise to her sincerley as he hasnt done anything wrong (yet). So ML hurt her, cant apologise and then they fall in love (im guessing in the end) all while she still has those terrible memories (years long trauma doesnt disappear with an i love you).