It is very understandable that you and your family are scared. It's even understandable that things feel like they are falling apart. However, I see a few reasons to be hopeful. First, you have been ill on and off for 12 years now and are still here to talk about it! You have made it this far without a diagnosis and without treatment. Just imagine how much better things should be if you are able to get a firm diagnosis and treatment at your upcoming appointments.
Secondly, it's more of a perception change than a reality change should you be diagnosed with lupus. You are still you, no matter what you are diagnosed with. A diagnosis doesn't change anything really, other than afford you the chance to start treatment that will help. This is the way I try to look at all the tests when I go through them. What's there is there, and I'm just going to find out what it is and start tackling the problem head on!
A word of warning - unless your GP ordered blood work in advance of this appointment - you will very likely NOT get a diagnosis at your first visit. Depending on what the rheumy sees at the appt, hears about your medical and family history, and the results of the bloodwork he orders after your appointment... then at the 2nd appt there is a better chance of a diagnosis if one is warranted.
Best wishes, and please keep us updated and let us know if there is any way we can be of further assistance. (((hugs)))