Dad replaced mum after 5 months

My mum died Oct last year and less than 6 months later Dad is in a relationship with the Ukrainian refugee I helped move in when was alive. I really want him to be happy and she is a nice person but I can’t cope with hearing about it or seeing them. I live next door as I helped care for my mum for many years as she had Parkinsons. I feel like I’m losing dad because I can’t bear to hear about the relationship which he is so happy about. It was our family home and I feel like we have been replaced by this woman and her daughter and grandchild. I don’t want to lose my dad but feel I need to protect myself as it hurts so much. It seems that I am not alone so thanks all for sharing. Any advice on how I can protect myself and not lose my dad would be gratefully received .