Is this really the right community to ask for a response?
Regardless, you don't actually need to say anything, just be there... and be there a lot.
Just be in here presence as much as she allows you.
Force her to drink a lot of water, no pop or candy, she has to eat even if she doesn't have an appetite.
You might have to argue with her for her benefit, since she has cancer, she can't be doing that meal skipping shit.
No fried, processed or fast food... or very limited amounts.
If you must have fast food, it needs to be along the lines of Subway (still isn't healthy, but better than McDonald's).
Ice cream is ok, but don't let her be a glutton about it.
Basically, as her boyfriend, it's your duty to keep her honest about her health.
She might try to dump you, ignore that shit, it's just her emotions getting the better of her.
If she uses the, "Do you think you're my fucking dad or something?" line, you reply as, "No, I'm the guy that fucking cares about you and wants you to live for the next 70 years."
Don't allow her to be insecure about the side effects of her treatment. If she's legitimately feeling sick, let her rest. If she's just embarrassed because of hair/weight loss or swelling, take her ass to the movies, call her beautiful and lick her face often. The personality you grew attached to is still there, it may be recessed, but it's there. If you allow her to push you away, you're an asshole and her personality may stay permanently recessed, she'll hate you forever, may become severely depressed and you'll always have a guilty conscious.
... I think that's it.
