Although I can't speak to the specifics of each disease here, this article can give you some general advice for now, to get the ball rolling on helping you find ways to de-stress and process all of this, and you can explore further resources afterward. (This way you won't be overwhelmed by too much information.)
First, you may want to get with a good support group. There are all kinds of support groups available for people suffering from various conditions, and if yours is rare, you may find benefits with a more general support group for those dealing with serious illnesses. This provides the benefit of connecting with others who are experiencing feelings that are similar to yours at this time. Also, getting to talk about your experience in a welcoming environment may also help a lot. (I encourage you to find support in The Stress Management Forum as well.)
Next, I'd like to recommend some basic de-stressing techniques that can help you to get your body out of stress mode (turn off your fight-or-flight response, which may be chronically activated at this point, understandably), and help your body maintain a healthier balance. This article can provide resources and information on breathing exercises, Progressive Muscle Relaxation, and other quick stress relievers.
I would also recommend maintaining an ongoing journaling practice. Processing your feelings on paper, as well as planning new courses of action, has been shown to have healing properties, and can help change your outlook as well. Here's an article that more fully explains the benefits of journaling, and can get you on the right path.
Also, many people faced with serious illness have found solace and real help in books written by Bernie Siegel, M.D. ('Peace, Love and Healing' is a great one), and Dr. Andrew Weil has a lot of information for healthy living in general.
In addition, visualizations, positive imagery, and positive affirmations can be other helpful tools in focusing less on what scares you about the future, and more on hopeful outcomes. This may be very difficult right now, but it's a good direction to work in. Dr. Siegel writes about this in depth in his books, but these articles provide a good start.
Finally, this free e course in low stress living can provide a good framework for leading a healthier lifestyle while you're going through this.