Feeling selfish guilt

I just wrote an extremely apologetic and long email to my friend who I saw only a few hours ago. Her mum is extremely ill with secondary cancer, and me, said friend and another friend went for a coffee and a chat together. This is the one thing that completely numbs me as to what I should say. Ok, maybe that's wrong.. I know what I 'should' say. But this somehow doesn't feel enough for someone who has been there so unconditionally for me (even though I'm so fucking anally retentive and show about as much emotion as... well, someone who doesn't show much of them). This thing is so much greater than what she has helped me through that no words seem adequate.
I don't know how to tell her that I care, and I am here for her other than to say it in such layman terms. And I'm feeling bad for not knowing how to tell her in a more poetic (if you will) way. And I don't know what to say. So as she and my other friend sit talking about how her mum has lost nearly all of her hair through chemotherapy, and how she can't stand to be near anybody for a week afterwards because any smells make her vomit, I am speechless. I, myself, cannot grasp what this means. And I find it impossible to imagine what it must be like for her and her parents.
Yet at this moment I'm also feeling selfish for feeling bad about how I feel. Get me? I just... ugh. I'm not good with words, and I'm not good with people. I just want her to know that I care.