Hi and welcome to the forum.It seems to me that whatever your dads reasons for behaving like that is something you will never know.The only time I have heard of something similar was a long time ago when a friends dad had Alzheimer's and his mum started seeing someone else.When he became too I'll to remain at home and was moved to a care home,she even moved the boyfriend in as a lodger.She visited his dad and had a double lifefor a few years before his dad died.The relationship with the boyfriend only lasted a couple of years after. My friend was angry and upset but accepted in the end that this was the status quo,even formed a light friendship with the guy.Afterwar her explanation was that she grieved for his dad long before he died but was so terrified of being alone she was looking for a replacement so that would never happen.in a way.She had become a nurse cook and cleaner but ,not a wife anymore.and here was this guy treating her as her husband used to do.Took her for meals,remembered her birthday,etc etc.
Of course you miss your mum.No one can ever replace her and it would be a foolish woman to think she could.
Its awful that you don't have your dads support but your grief is about your relationship with her not his,your good memory's of her are yours and they can't be taken away.None of us know your dad or why he behaved the way he did,but all of us know about grief. I lost both my parents a few years ago and it was losing my husband that brought me here,but I know there are others here nearer your age who have more recently lost their mums so I,m sure you will find support here to help you with that loss.