Indigo
I echo Sandra's words and will add a few of my own.
Right now you will be going through so many emotions that they all start to run into each other. Feeling that you are alone even when you are surrounded by people is something I still get. No matter how hard I try I just think nobody "gets" me in that particular moment. It does pass and I know many other people who also feel this way so it is a common thing.
You are almost certainly beginning to grieve for your Dad, this is because you have accepted the inevitable so it is also absolutely normal, you really do not want to see your lovely Dad in pain or suffering and are worried that continuing the Enzalutamide will just prolong his suffering.
You sound as though you wish he could just slip away peacefully, but then you might also struggle for feling that way . Please do not be afriad of these feelings it is because you love him deeply that you simply cannot rationlaise or deal with this huge conflict of emotions.
You are also right, that old saying about shooting horses does make us think that we are so cruel to allow our own loved ones to suffer so much.
Your Mum will be going through her own and likely different emotions so you will both need to just bear with each other as best as you can to provide as much support as ever for your Dad. In time those emotions will become the same for you both and it will be easier for you to then support each other.
We are all here for you if you need to share your own personal pain and anguish.
Thinking of you
xx
Mo