Well.... i can't really argue without spoiler , my only problem was he should have trusted her more and told her about everything instead of leaving her in the dark but in their situation at the time it couldn't be helped and Sian thought he could handle everything on his one but at the end elena still died without knowing a thing.
I tried to explain it as much as i could without spoiling, sorry if this confuses you ,and before i read the novel i totally agreed with your point but after reading I understood his actions and he kinda grew on me
So at first he hated the real veronica since he really traumatized her when they where kids, so he thinks she's psycho (which she is) .They get married for political reasons after Cecilia died .Mind you he still didn't like veronica but the current veronica is our mc elena and elena fell in love with Sian so she starts acting nice around him and trying to win his love but Sian still thinks its the real veronica and ignores her cuz he thinks she's planning something and has ulterior motives so he doesn't trust her cuz people can't change easily. So they end up sharing the first night together that's where Sian realizes its not the real veronica so he investigates it then ends out finding out about elena's true identity and that tge real veronica is still hidden somewhere. He them falls in love with the elena and wants to protect her but he can't show his real feelings cuz if he did the grand duke (veronica's father) will most likely take elena away and replace her with veronica.(he has spies all around the palace) .So Sian tries to strengthen the imperial power to go to war with the grand duke cuz at the time they were still weaker than the grand duke. Thats where elena gives birth, sian was actually really happy about ian but he couldn't show it cuz there were risks about elena and ian being hurt by the grand duke that's why he couldn't show his feelings.
So when he says everything was a mistake elena fell out of love with sian and resented him. Sian was trying to get elena and the baby to flee away whilst he went to war with the grand duke but elena didn't hear him out because she was still pissed off
Some time later the real veronica show up in elena's room holding ian and Sian screamed at her asking where elena was he then took away ian who was crying in veronica's arm he stopped crying when Sian held him .Sian then rushed off to find elena, when he got there it was already late and she had died already cuz the grandduke killed her ,he them broke down but was comforted by ian and he tells ian he would protect him cuz he's the only thing elena left in this world . He later on goes to war with the grandduke but the results weren't explained so we don't know what happened.
Thats basically what happened.
There are literally no excuses for what the prince have done wrong (even if it was in the previous timeline). I read that “the prince have always loved her but just didn’t know how to express it”, but that’s just flat out bullshiii..whack. Don’t taint the name of “Love”. If you love someone, you shouldn’t have made them suffer so much especially when neglecting them while giving a mere stranger the attention she’s always yearned for. Characters like these are unforgivable, I hope Elena finds her true happiness. Try to change my mind if you can, but forgiving the prince is a big NO.