Mum’s moved on
It’s selfish
It’s your mum’s choice. She deserves to feel happiness again and being upset with her is slightly selfish. If it was an ongoing battle before he passed she was mentally prepared for it to happen. If someone is bringing her happiness after such a loss shouldn’t you support that and feel happy for her? Or would you prefer her to sit at home, all alone, grieving still?
Different grief
It’s probably extremely hard for your mum, especially if her children have spouses. She is on her own and the very person she would turn to in a situation like this is gone. You’ve lost your father, but it’s a different grief losing your husband of many years. You both had a different kind of relationship with him. This person may be no more than a companion at this moment.
New phase
Sometimes things happen in a way that can’t be explained. Your mum grieved for eight months and will never forget your dad but this is a new phase in her life and she is testing the waters. Allow her the dignity of deciding what is good for her right now. This may or may not go somewhere but you have to be ready for however it goes. Leave the paths of communication open. Perhaps make an opportunity to visit with this person to also get to know him. Be a support for your mum as she explores this new normal.
Your feelings are valid
Grief has no time limit, so your feelings are valid. However, remember this is your mother’s life and her decision. The love she had for your dad will not be diminished nor forgotten but her need for companionship other than from her kids is real. Be happy for her.
Walk in her shoes
Would you prefer that she stayed sad and lonely for whatever time she has left? You don’t know how it is until you walk in their shoes.
Be proud
Be proud that your dad gave her the confidence to move on. She started grieving before he died if she nursed him for some time. Be thankful she has found happiness again.
You can’t be angry
My husband’s wife passed away eight months before we started dating. Five years later, we are married and have two kids! Everyone grieves differently and you can’t be angry at your mum for moving on. She has every right to move on and be happy – it doesn’t mean she loves your dad any less.