Hey there! I actually started experiencing my symptoms a few weeks after I graduated with my RN degree. I took boards in pure misery! I sometimes wonder how I passed! Anyways, I won't lie, I wanted to give up there in the beginning when things first started and I didn't have a diagnosis. I was pretty sure I had fibro though and was encouraged by a counselor to continue down the path I had worked so hard for. I am so glad I did! I now work at one of the best Hospitals in the nation. I do straight 12s, and yes sometimes I am tired and achy afterwards or during but I find when I am there I usually feel fine save for a few occasions. I think the distraction of work is a good thing for me.
Keep in mind as an RN you can go down many avenues, you don't just have to work 12 hour shifts in a hospital! There are tons of opportunities!
Fibro sucks and there really is no getting around it but don't let it limit your desire to work in the field. It can be managed.