Big problem with my girl(s)

I've got a big problem.
I have a really sweet, nice, good girlfriend for allmost a year.
I love her. That's not the problem.
The problem is I know a girl, I like very much too.
She's totally different different to my girlfriend.
A long time, I thought she'd be a level too high for me, but the last days, I realised she isn't. We do many things together with friends, at school, so on...
She likes to do things with me.
I'm happy to be next to her, but I'm also really happy to be next to my girlfriend.
I don't know, if there'd be more between us, if I'd try.
I don't want to destroy the friedship or the love. But when I'm next to my gf I sometimes think of her and the other way round.

I don't know what to do now, I'm really despaired.

i'm the treadstarter:

that's the problem, normally I would set my gf over everything else, but nowerdays we have some problems, no quarrel, but problems. And we cannot do anything, then wait, wait, wait.
Cause we both know the problem and cannot do anything against it.
(Her health...)

And then on the other side, there's this totally healthy, cute, sweet, intelligent, sexy other girl...