"The hardest part of my parents' divorce was the moment I saw my dad with his 'new son'."

My parents’ divorce was messy.

It was one-sided, surprising and swift – a break up that left my entire family reeling and my mother in pieces.

For those initial few weeks, every day presented new challenges; fresh revelations about the actual state of my parents’ 32-year long relationship, their weaknesses and failings. The trauma of their breakup was one that saw my sister and I band together, becoming each other’s crutches when it felt like everything and everyone around us were crumbling to dust.

When a divorce isn’t amicable, you see ugly sides to your parents you never knew existed.

Despite us ‘kids’ being adults by the time of their separation, the pain was no duller. Finding out about our dad’s “new girlfriend” a total of two weeks after he left mum was confusing and crushing.

And yet, with time, it got easier. As these things do.

Eventually, the lights in the kitchen were no longer on at 3am. My mother could sleep without either of us beside her in bed. The house we all shared together was packed into boxes; the furniture sent off to op-shops and second-hand stores, tables and armchairs given away to families who wouldn’t be reminded of my dad’s deception just by looking at them.

My sister and I learned that our parents are flawed and vulnerable, and forgave them for the mistakes that wedged our home in half. We repaired the broken pieces of our relationship with our dad. We put on tough faces as The Girlfriend began appearing at family events. We sucked it up.

My dad walked out four years ago, but only now have I realised what is most painful about him leaving us.

The most painful part of my parents’ divorce is not hearing the private details of their marriage. It’s not the legal battles, or affairs. It’s seeing him with his new, shiny, younger family.

It’s seeing my father – the man who called my sibling and me his pride and joy – trading up to a life and son that do not belong to him.

The Girlfriend and The Girlfriend's Son are now the centre of his world. They have their own traditions, now. Their own inside jokes. I hear about it endlessly on the phone. His real children are old news; The Girlfriend's Son is what brightens my father's existence. He delights in everything the boy says and does, and I watch on like this is something from an alternate universe - one where there isn't really room for our old family, the original family, anymore. Not really.

My jealousy makes no sense - I know that. It probably sounds petty and cruel that I feel this way about my dad's happiness. After all, I am a 25-year-old woman, and this is just a boy. A boy who loves to sit on my dad's lap, to ruffle his hair and play with his hands. Who just wants to play in the pool and have my dad watch him do dives.

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But when your father grew bored of the life you created as a team, when he chose to leave it so abruptly, it hurts so much to see the people he chose instead.

And whenever I see that boy on top of my dad's lap, that's all I can feel - sadness. Sadness that this is what my dad wanted, and that he's happy, but it's the furthest thing from what I ever expected.

Am I being totally unreasonable to feel this way, or have you been in the same position and felt the same? Let me know in the comments...