Its the "How" that I need help with!

I'm new here too. I'm a 42 yoa male, the oldest of three, I have two younger sisters and a wife & 4 kids of my own. My dad was diagnosed with Melinoma late in 1998. he was told it had been caught early and underwent a chemo treatment (unknown what. It did not seem to be a big deal)...afterall He's my dad...gonna be here forever!!. I had my own family to take care of. The treatment went well they said (as I knew it would). One year later he has a heartattack and during Xray, find what's believed to be cancer. Yep! its back 14 months to live, nothing they can do! Dad didn't accept that answer and went elsewhere MD Anderson...They said Yep we can help, "we'll cure you or kill you" is what he told us the doctor told him. I wasn't worried...he's my dad...gonna be here forever. He underwent a very aggressive chemo, multiple transfusions, then surgery..I wasn't worried...he's my dad. The treatment worked and he was clear of the cancer...God worked a miricale through the doctors and staff. Now 5 years later, during a follow up check up they find its back, and put him through another "Chemo Light" as he called it, for 6 weeks. The "Chemo-Light" did not work so now hes going back to the hard stuff. Now I'm worried...he's my dad and I know he won't be here forever...
I don't know how to deal with watching him suffer...the vomiting,& weakness. Its like he's turning into an Old feable man in front of my eyes. I only had to watch a very limited amount of time the last go-around. Thank God for my mother and sisters who stayed with him nearly around the clock. I didn't have to watch Or participate in his treatments all that much before because He needed me to keep his small business open for him during the day. So while he was in the hospital during the day, I was Hiding from the cancer effects by running his business and at night I was working my full time job till it was time to open his business the next day. I worked those 20 hour days and was thankfull I didn't have to watch him suffer. My own family suffered because I was gone so much, but we survived .... I can't hide now like I did then.

How do you be of help, how do you be the strong dependable Son and older brother and father to your own kids that everyone expects when you can't stand to watch this. I'm not questioning God, my faith OR asking anyone as to WHY..I just need to know HOW!!!!! Afterall He's my father and He's not gonna be her forever. I can't hide this time and this time around probably counts the most. I can't let him down, but I just don't know HOW!!!!!
I'm so sorry for the length...Ive just never let this out before to anyone