Dear tsm, you ask why you are unhappy, and to me the answer is fairly plain.
Your mother has just been diagnosed with dementia, so obviously you are worried about her. How long can she cope by herself? Will she need to go into a care home, and if so, will you be able to persuade her to do so? And possibly a lot of other questions as well.
Having a parent who is diagnosed with dementia or a life threatening cancer, obviously makes the query, "Will that be me one day, too?" rear its ugly head in the back of our minds.
And then to cap it all, you and your husband have different ideas about what to use the free time retirement brings for.
You want to do things, go places, while you still can, he is apparently happy to be a couch potato.
As I do not know either you or him, I cannot suggest how you can resolve this. It sounds a bit like the unstoppable force meeting the unmovable object - but I do so hope the pair of you can find a compromise you both can live with.
In your place, I would start by talking to mother. Find out exactly what her prognosis is, and what she wants to happen. Suggest that she has a provisional power of attorney drawn up and re-reads her will NOW. She needs to know that if the day comes when she cannot manage her affairs someone she trusts will do so according to her wishes. But to be able to do so, the person holding the POA needs to know what she wants.
And while you are about these rather dreary tasks, attend to a POA for yourself and your will.
I know, I know, none of us like the feeling that we have lived the longest part of our lives, but both my husband and I were thankful when he was diagnosed with an agressive cancer that he died of three months later that we were able to talk things through, and that the legal matters were sorted, and I knew what sort of funeral he wanted. This may sound morbid, but having it clear can make the end of this life easier than it othewise would have been, both for the person leaving and those remaining.
Remind yourself that none of us know (thank God!) when we will die, so enjoy the rest of your life. Retirement can be difficult to get used to, and it cannot make it easier that your mother is faced with a serious heath issue.
I don't think any of us really ever think of ourselves as OLD, although having just mown a large lawn for the last time this year I KNOW I am not young. Lawns were easier to mow when I was 33 rather than the 73 my date of birth insists on, but life is still worth living as I hope you will feel once you are out of the "have I really reached retirement age doldrums".