First off, it's wonderful that you love him so much and are willing to stay by him. So many people run where there is sickness involved.
My mom has Lupus and she was diagnosed about 28 - 29 years ago. So while Lupus is a very ugly disease, I do believe that so much of how it (and many other things) effect each person is attitude towards it. My mom was always the strongest person I knew. I couldn't think of anything she couldn't do and I never thought this disease would get the better of her. From what I have read on this board, I know that I - we were lucky to have so many years of virtually nothing really wrong. She kept a very positive attitude, took the medications and kept up with what was going on her with. She had a lot of time where she was in remission and she still followed up with her Lupus specialist.
You have to take time to get used to the fact that he is sick, yes. But try not to let it be the first thing you think about when you think of him or look at him. The two of you could have a very long and happy time together with children and the whole nine!
A huge factor in his health is to keep the stress down in your lives is a huge factor to keeping the flares to minimum, proper diet, and enough rest.
I would also recommend getting counseling for yourself to help you deal with the sense of loss you feel. Remember the 5 stages of grief and work through them. And you have to be strong for him as well.
I hope this helps and you need to talk to someone, please feel free to send me a message.