Hi OP,
What I’m about to write isn’t to try and make you feel bad but I was in your partner’s position many years ago so I thought it might help you understand it from the other side.
I had a DD who was 3 when I met my ex. My DD understandably was (and still is) the most important thing in the world to me. She has always been such a well behaved, lovely child... even at 3, no tantrums, good manners etc. People would always comment on what a nice kid she was.
My ex had literally zero experience of children. He was a late “surprise” baby for his parents and his 2 older brothers are 12 years older than him. His niece & nephews therefore were in their early 20s when we met and I was only 26 myself so there were literally no children whatsoever in his family.
He was a good man. He loved me very much and appeared to really care for my DD too. Her dad isn’t in the picture (lives in Australia, us in the uk) and I honestly thought we’d met our “happy ever after” man.
We moved in, got engaged and I fell pregnant with our DS quite quickly (failed coil, unplanned). Whilst living together I noticed him getting irritated by my DD more and more for just behaving like a child.
He was annoyed at her wanting to sing, or set up tea parties in the living room for her dolls, or her climbing into my side of the bed in the morning.
He resenting the amount of love and attention I showered on her because he’d never been in a relationship where he had to “share” his partner before.
I tried to be understanding but I remember clearly a few incidents when I was pregnant where I watched him physically recoil from my DD when she tried to be affectionate towards him. I watched her lean into him for a cuddle and the look of revulsion on his face was clear to see.
I knew there and then that our relationship couldn’t work. It was even harder when our son was born and I saw him overwhelmed with love for him but seemingly brimming with annoyance for my DD who was adjusting to life as a big sister.
She had been my one and only, and she was overwhelmed with love for her baby brother but also (bless her) concerned for me. “Stepdad you have to be careful not to put the blanket too close to the baby’s face because it’s dangerous” she said to me one day.
My reaction was immediately to find it lovely she was so caring and remembering the things I taught her about the baby. My ex decided it was patronising and it resulted in a huge row with him saying “I’m not having a 4 year old telling me what to do!”
He perceived everything so differently to me in the same situations because he literally had no clue about children or how they behave.
We split up when our DS was only a few months old. My ex at the time didn’t realise just how damaging his rejecting behaviour was being to my child’s self esteem.
And at the end of the day, I’m her parent and it’s my job to protect my child. I had to leave so she wouldn’t feel “less than” in her own home.
OP you cannot fix what you unintentionally did, but you can make amends now. My ex and I did not get along for a year or two post split... but one day we had a frank discussion when our DS turned 3 (the same age my DD was when we met) and my ex said “I look back at what DD was like now DS is 3. She was a really good kid and I realise now I was wrong. I’m sorry”.
I cried, he cried, we had a hug. It was a turning point in our relationship. We became friends from that point on and our coparenting relationship is great now. He’s even built a nice relationship with my DD (who is 10 now and DS is 6.5) and has her for sleepovers with her brother occasionally or he’ll include her in days out.
Unfortunately our romantic relationship never recovered. I’m actually due to get married to lovely DP who I’ve been with for 5 years.
But you can make this right with your child’s siblings and your ex. You can all get along now even though you’re not together.
Acknowledging how you treated his children and apologising (even though it was unintentional) would be a big step in healing some wounds.