Hey Elena --
I feel this... I flew one way from BC to Ontario at the end of November to help my mom with stage four stomach cancer and during the five weeks before she died it was incredibly isolating, overwhelming, crazy-making, sad, stressful, boring -- all of the emotions. I posted in this forum about feeling like I was in a personal hell cuz I didn't want it to end but at the same time I needed it to end, it was very intense, so I get you. This is not easy and no one will really get it until you're in it.
Because I knew that once this was all over I would be devestated that she was gone and I would be crushed if I hadn't appreciated the time I had with her, I was desperate to stay present and relinquish control from the not-knowing. Something that helped me with that was finding a few mantras that I could journal and re-journal. A few that resonated with me were, "I trust in this process. I trust that this journey is unfolding at the pace it should. I trust that I will look back and say, 'Everything happened exactly as it should have.'" It really grounded me and stopped me from swirling.
TBH, I don't know if there's a way to experience this without it being really, really hard, so you may need to just focus on your own basic self-care needs (so at least two good meals a day, a good sleep, one good walk a day, phone calls with friends or your spouse) and just managing the rest as it comes. Know that you will get through it. Know that you BEING there is the best thing for you and your dad, even when it's hard. Know that you will not regret being there for one moment.
And, yes, watching the deterioation is heartbreaking and overwhelming. I was really scared at the beginning, seeing how much weight my mom had lost, seeing how weak/exhausted she was when she walked, watching her experience pain, trying to wrap my head around her decline. I recommend crying when you need to cry. Hopefully you can cry in front of your dad and he can hold you. Let him cry, too. Those were some of the moments that brought me and my mom closer together. Eventually I came around to acceptance that this was what death looked like, and it was going to happen whether or not we liked it, and it was f*cking awful but it happens to all of us. This is what death looks like. Given that your dad is going through chemo it sounds like he'll have more time than my mom, but, yes, you are experiencing the dying process firsthand and it's pretty overwhelming, no two ways about it. <3
I don't know if any of this helps, but know that you're not alone, and you're heroic for leaving your family to be with your dad. It's courageous, selfless, and wonderful that you are there. You are doing a really, really good thing. Take care of yourself and keep posting in here as you need!