Congratulations to all those caregivers who continue the struggle.
Caregiving includes things like taking over all the household functions the sick person handled, at least as best as one can. In addition to the difficulties in doing things you don't really know how to do to their satisfaction, there is the feeling of uselessness that they can suffer. All you can do is your best, then be as consoling as possible.
With chronic severe pain, there is disruption of sleep, both from the pain and any drugs used to ameliorate it. Trying to work full time can lead to week after week with insufficient sleep.
In situations like that, at home stuff you can do like reading and watching television and posting at peculiar hours might be the only respite you can afford.
But hard as it is, and as much it's not what you wanted for your life with that person, the alternative, an end to the caregiving, as frustrated and exhausted as you may get, really is not what you want, or need. Do your best to remember that.