Liz,

Should I just get on with what I have and build a life outside my marriage, which is what I try and do?

You made a deal; you get financial security, he gets care as he declines in age.  I wonder what if you get sick before him?  Will he take care of you?

Now you realize that you want more than that for your marriage.  You want sex and companionship.  There are those couples have an agreement where the spouse can go outside of their marriage to find that.  You might want to consider that arrangement.

Regardless you need to make a choice in how you want to live out your remaining days; with regret and loneliness or with some sort of life that involves living life.

Mark