I feel with and for you. It is heartbreaking. Both my parents had dementia by the end although my father stayed loving and kind towards me and my mother was always demanding and a whack job. It is hard to go through. If they don’t get aggressive and hostile with it, it is easier to take. And luckily both of them pretty much knew who I was until the very end so that helped.
I would say, assuming they are being cared for somewhere and you are not responsible for that aspect, go when you can to visit but don’t feel you have to go every day. Make the visits short and sometimes bring another person so the two of you can talk and provide stimulation but it is not all on you. Be loving but be brief. If they are hostile, leave. If they ar in care, try to ensure that they are being well looked after. A family member who is alert can protect them.
One thing that helped me with my Dad is that we would often hold hands when I visited and his grip remained strong and loving.
And try to keep the good memories alive. They will sustain you later after the person has died. and, if you do feel relief or numb when they are gone, don’t beat yourself up about that.