In concert with what GalaGirl is saying, I believe that the core of your difficulties right now is the challenges of taking care of your disabled father. Nothing is wrong with your relationship with your boyfriend, except that you and your boyfriend are not getting a fair chance to spend quality time together, without the burden of caregiving. Of course it is not your father's fault, but it is not your fault either.
I would still go ahead and get that book and have a look at it. But in the meantime, I would have serious talks with your boyfriend (and father too I suppose) about what can be done to allow you and your boyfriend to have a little more of the life that you had before. Again as GalaGirl said, this means, at the least, finding someone to care for your father for periods of time while you and your boyfriend go out on a date.
I think another thing you have lost is a special, private space to share with just you and your boyfriend alone. Your home and your father now come as a package. This is a serious loss to what you and your boyfriend need and deserve to be with one another as a boyfriend and girlfriend should. At the very least, you need some "motel time" (either in the day or at night, depending on what you can arrange).
Above all, you need time away from your father. You need a break. It is not your father's fault. But it is not your fault either.
Is it time to consider longterm care for your father, at a facility that specializes in such care? Can Medicare/Medicaid help? These are only questions, and you need not tell me the answers if you don't want. But the questions need to be asked. Your primary relationship is being damaged by the situation, from what I can so far tell.
It's no wonder at all that your sexual drive for your boyfriend isn't what it used to be. Of course you are feeling more sexual drive for your old-time friend. When you are with him, it's in a pleasant environment where you and he can spend some quality time together. That's what you and your boyfriend don't have. So in my opinion, you shouldn't have to feel guilty that your sexual feelings are "going in the wrong direction." All's that's going on is that you (and your boyfriend both) are going through a period of intense stress, without the advantages that a romantic couple naturally needs.
I hope I do not hurt your feelings or offend in speaking so forthrightly. I just feel really bad for you, having heard more about your situation. I know there are no easy answers. You'll just have to figure out what you can do as you go along.
Whatever you decide in the "short term," the interesting twist in the story, that you're now contemplating an open and/or poly relationship, will probably remain a part of your life in mind, long after you "solve the father problems" as well as you can. This is a good thing. Even if you decide not to open up your relationship, you will have still opened up your mind to a new thing, and learned about "a new world out there." Knowledge is always beneficial. If nothing else, it will give you an appreciation for people who are poly.
And, it has already given us, on this website, a new appreciation for you. You are a strong person. You have undertaken a great burden. Take care of yourself too. Take care of your relationship with your boyfriend. And please continue to keep us posted here, and talk to us about your thoughts, questions, and/or how things are going.
With respects and regards,
Love means never having to say, "Put down that meat cleaver!"