One book that looks promising and is highly rated is The Orphaned Adult by Alexander Levy.
I'm not a psychologist, but it is possible that in some way you mourned not having a relationship of mutual interest and admiration with the deceased parent long before they died. And now that you are having to care so much for the remaining parent, and now that you are watching them be so disinterested in your own children, the old wounds from them not taking more supportive interest in your young self now get raked open regularly.
I don't know if you will find this helpful or not:
One time in middle school I remember asking my Dad "Dad, you know my team is really good, we might be undefeated this season, will you please come see us play a game? Some of the other parents come to all of the games . . . "
And his answer was one of the truest things he ever said to me:
"Well, you know, your mother and I have our own things that we are interested in doing. We think it's great that your team is good this year, but we'd just rather focus our time on our own interests."
That's it in a nutshell, and it can be applied to anything I was working on. Bottom line: their other projects were vastly more important to them than being fully supportive parents. Neither of my parents understood that an important part of showing your children you love them is supporting them in their endeavors, whatever those are, especially when they are struggling. My Mom cares even less about our interests and accomplishments. She still routinely belittles my brother for not finishing college, for example, but she didn't offer to pay a dime for it (he dropped out when he just didn't have money for his last semester of tuition), she never visited him when he was in college, she never went to even one of his matches, and he is professionally more successful than she has ever been. Yes, he struggles with it. I struggle with it too. Once she passes, it will be forever unresolved, but I don't think it can be resolved. They are who they are.
So, both of my parents are extremely ego-centric. They didn't stay together because they didn't admire each other's creations often or loudly enough, honestly. They aren't interested much in visiting with my children, or any children, because children are also ego-centric, and so most children don't spend any of their time telling the adults around them how wonderful they are. Contrast that with the adults they surround themselves with: people who fawn over them about how wonderful and talented and skilled they are.
(Hugs)