Diet can be a factor for some but many have found it has no effect. One of my early doctors was a well known clinical researcher for lupus and said that having a good, balanced diet was beneficial to anyone. He also said that some patients benefitted from an anti inflammatory diet like the one that Dr Andrew Weil and others advocated. There are also some foods that lupers should avoid. Nothing has been proven conclusively about diet and lupus except for a few foods that should be avoided.
Many follow a gluten free diet which is really only recommended as a treatment for celiac disease or intolerance. There is probably a placebo effect for some when going on it but again, nothing has been proven by medical researchers.
My personal experience is that diet did hot help or hurt in my case. I did modify my diet slightly to make it a little better but nothing draconian and 8 years into lupus I pretty much eat what I did before getting sick and am in full remission.
You also have to take into account what meds you take. For instance, if you take prednisone, reduce carbs in general and avoid simple carbs if you can...at least any additional refined sugars. Of course, that is good advice for anyone and I have been doing that for the last 50 years anyway.
You are a little younger than my kids but I can tell you that after a life threatening and crippling cases of lupus and polymyositis (lost 40 lbs of muscle in a week which left me quad with a feeding tube) I lead an active life even in my 70s with some restrictions. Even with those I am more active than most my age.
Assume you will be OK, live in the present, manage your healthcare aggressively, and remain optimistic about your prognosis. If I would have listened to all the BS and even professional forecasts I would not have made it let alone be able to live the life I do.
PS - you find more negativity on the internet and many of the successful people stop posting as they get better. That is why I continue to post....to give people advice and hope. Won't happen for everybody but it can happen for some, like me, even for those with extreme cases.
MCTD (SLE Lupus, polymyositis, scleroderma). Diagnosed 2005. Kidney, liver, GI tract, dysphagia, raynauds, barretts esophagus, quadriplegic in 2005. Recovered and now active in downhill skiing, golf, hiking. Meds: amlodipine, benezapril, omeprazole, potassium, folic acid, vitamins, maxide.
Remain Optimistic and you can overcome.