I know how you feel too
I have felt the same resentment, but not at having to look after my wife with her Dementia, but at the condition having robbed her of the time of life she should be enjoying after a hard working life.
My wife is 8 yrs older than me, has no living family and only me to care for her, and i'm the only one she wants to care for her, no carers or outside help coming in at all.
Up until last July she had struggled to cope during the day while i was at work, but it eventually became to much for her being at home alone, plus having to cope with neighbour trouble as well, that she asked me to give up work and be with her full time, which i did as i did used to worry about her while i was working and how she would be coping.
It was my choice, and i am more than happy to devote the rest of my life to look after her and take care of her in any way that she needs, even to those personal bits that some people feel that they can't cope with doing.
As it says in the marriage vows, "in sickness and in health, till death do us part", and i am fully prepared n more than happy to do what needs to be done to look after her.
I know that its not for everybody, devoting the rest of their life to looking after their partner, who most of the time won't know who they are, and no one can blame them in any way for the resentment and in still wanting some life of their own.
You need to do what's best for you, as well as for them.