When you think things can't get any worse (cancer related)

They do.
I've posted a couple of times that my Dad has cancer, it's bad and it has spread. It started off in his bladder, then despite surgery and a year of chemo it still spread to his lungs and his bones. He has secondary cancers all through his spine and several of his vertebrae have broken. Ten days ago I went to an oncology appointment with him where they told him that there is nothing else they can do for him. That was tough enough, he was crying and begging them to try more chemo...but they told him that any more chemo would shorten his life rather than extend it.
He was admitted to hospital the day after that and then moved to a palliative care unit (hospice) last Monday.
He is having a great deal of pain and they are giving him the upper limit of the heavy duty painkillers, he has a pain medication pump going constantly with 3 different drugs and he still has pain. It is horrible to watch. He's lost so much weight and he's just fading away.

Mum and I have been there with him everyday (I've taken two weeks off work)...except yesterday I woke up with a sore throat and a cold.
So I've taken time off work to be there, and now I can't go in. Mum has to do it on her own. How f'ing unlucky is that?
I'm not *that* sick, but he has over a dozen tumors in his lungs so if he catches a cold, it could quite literally kill him, even coughing would probably cause more bones to break in his back or ribs.
So I have to sit at home and not see my Dad for some of his final days and my mother is there all day on her own.
It's just so unfair, I know logically that I can't go in, but the guilt is hitting me hard tonight.
His cancer, his pain, his dying, it's all just horrible and way worse than I ever thought it could be.