I'm currently looking for a chronic illness that a teenage character has been dealing with since childhood and has given her a lowered life expectancy (namely in her twenties, thirty at most). While there would be a lot of concessions to her health, she would still attend a normal high school and generally be able to maintain some independence (go for a walk with friends, learn how to drive), but I also want to avoid the appearance of a Soap Opera Illness where outside of the occasional hospital visit and sad sense of her diminished future, the character has no other complications. I've been leaning towards either something either heart or lung related, specifically one that requires a portable oxygen tank which would be manageable but require constant vigilance and provide an easy visual cue to her peers and the Main Character that this person is ill (also it's one of the few medical apparatuses I know anything about, outside of inhalers), but I haven't managed to hit on something that meets the aforementioned requirements, namely:
+ occurs in childhood
+ chronic and eventually terminal, at this time giving her a life expectancy between 5 and 12 years from the start of the story (currently aged 17)
+ could require a transplant
+ currently allows her some semblance of high school normality with concessions
+ heart or lung related, preferably involving to long-term oxygen therapy; heart would honestly work best within the story, but lungs are what I am most familiar with and google has at least told me that both use oxygen therapy as treatment so...
The closest I have come across is Cystic Fibrosis, but only because it's the only illness that meets those requirements that I can find information on relating to childhood and teenage experiences. The terms I have googled are "list of chronic illnesses/diseases", "list of terminal illnesses/diseases", "chronic terminal illness/disease", "illnesses/diseases that lead to lower life expectancy", "illnesses/diseases that cause death at a young age", "oxygen therapy", "illnesses/diseases/conditions that require lung transplant", and "illnesses/diseases/conditions that require heart transplant", not to mention a few others I can't recall at the moment. I read all the results on the first 3 or so pages of each search query. I also checked 'chronic illness', 'terminal illness', 'heart transplant', 'lung transplantation', 'congenital heart defect', and 'life expectancy', in Wikipedia, and 'congenital heart disease', 'congenital heart defect', 'heart transplant', and 'lung transplant' in WebMD. I also terribly reviewed the Make-a-Wish website for possibilities, which makes me the worst person ever. Any help and links to information, would be greatly appreciated!