I'm sorry to hear of your dilemma. First of all, you are none of the things you are asking about.....not a terrible daughter, not selfish, not a bad caregiver, and you're certainly not crazy. You remind me of my uncle at this point in time, some different circumstances, but same situation.
My uncle comes from a family of 10 children. He never married and remained living at home with my grandmother. My mom (his sister) lives overseas from him, and we do what we can to help. My mom flies back and forth every 3 months to try and give him a break, and spends hours every day talking with him so he can let out his feelings. There are 4 other siblings all living 10 minutes within him and my grandmother. My grandmother is 94 years old and is in late stage dementia. To make a long story short, I will say that I think its one of the most horrible ailments a person could have. Because he was living in the house, it just sort of "happened" that he became her caregiver. AT the beginning, everyone would come and go, but as things got worse, and she needed constant care, the visits grew less often, and the bitterness and resentment grew to the point where people cannot bare to be in the same room together. One of my aunts has not seen my grandmother in almost a year, and she lives about an 8 minute drive from her. I feel so sorry for my grandmother....her mind is just tormenting her. She is so old, that they will not even offer her care in a nursing home, the only choice they are being given is a mental hospital. The thing with dementia though, is that "sometimes" the person, can be themselves, and that's what makes it such a hard decision. My uncle goes 3 to 4 days with no sleep, has so many health problems now of his own, because he has put himself last. The thing is, his family allows him to do it. Some may say it's his choice, but if you're the only one who will do it, I dont think it's that black and white. Because when you're the last one there, you end up being stuck with the final choice of whether or not to care for the person. Very difficult and heartbreaking decision. I dont know what's worse for him, his exhaustion, his depression, or his bitterness at the family who continues on with their lives, and gives them an hour or two a week of their time while he lives this "hellish" life as he calls it. I guess I'm telling you this story because I know that you probably feel backed into a corner, and angry because you are there. You obviously love your mom, and I can tell you, if I was in your situation, I'm sure I would feel the exact same. Not every one would make this choice, and it says alot about your character. Unfortunately, knowing that you're a good person, doesnt help pay the bills, doesnt help feeling depressed, or doesnt help your anger towards your siblings.
What is your mom's prognosis? I'm sorry about her illness, truly I am, as my mother has had breast cancer twice, and this particular illness has struck my family so many times. Are your siblings open to talking about this? Can you sit them down and tell them you need their help? Make a schedule? If they are willing to come and visit her more often, you could get some time to yourself. Please whatever you do make sure you do not get 100% involved so that you absolutely lose yourself. In watching my uncle, it is easy to do, and not an easy thing to fix.
I wish you all the best with this and hope things will turn around, and you can get some help.
Keep posting, this is a great place for support.