I guess I was fortunate in some ways that my mum died quickly after the cancer took hold so I did not have to suffer as you are.
I have friends now who are in a similar position to you and are suffering the agonies of the frustration that it brings. A doctors advice was to take a step back and look logically at the situation... realise there is nothing you can do but take it one step at a time.
Try to carry on with your own quality of life as best you can and carefully choose the time you spend with your relative.
That sounds O.K practically but 'feelings' are not always congruent with practical are they?. Best wishes and my thoughts and prayers will be with you, Mike.