Hi Kris, I've recently gone through a breakup where we were living together for 8 months. She didn't cheat on me (that I know of) but she wasn't supporting me. She was bulimic and emotionally unstable. I knew I needed to pull the plug, but hesitated for months for some of the same reasons. 2 years ago I was with another girl who did cheat on me. I'm learning the hard way that it's better to pull the plug when you see the red flags. Stress, especially relationship stress, can have a huge impact on your health. There are great girls out there with their heads on straight who will treat you right. Just because you have CF doesn't mean you have to settle for someone that obviously doesn't care about you. You're gonna kill yourself if you put yourself through that stress.
Also in my experience girls 18-20 usually haven't had to face that much in life. They don't know what they want and typically don't have to worry about all that much. The kind of things a CFer faces on a daily basis is hard for them to comprehend and thus hard for them to show support. There are exceptions of course. Anymore though I've been more interested in dating older girls. I'm 23, last weekend I went on a date with a girl who was 25. I told her up front I had CF and she wanted to see me. Older girls, especially those living on their own and working, just have more life experience. They like a guy who has similar life experience and isn't one dimensional.
20 is a tough point to be at when you're a guy with CF I think. You're looking for qualities that girls (and guys) your age just haven't acquired yet. It's not really reasonable to ask a 19 year old girl to wrap her head around a terminal illness she probably doesn't fully understand and can't adequately support. Be VERY selective. Watch for red flags and run when you see them. Take it from a guy who's spent literally $6000 trying to support a girl that cheated on him and lost 20 lbs during the breakup, then pretty much repeated that situation with another girl 2 years later. Be choosy. The wrong girl will use you and break you if you're not. From what I've seen with other happily married CFers, the right girl will be your rock, stick with you through hell, and make your life amazing.