My sympathies and condolences go out to you. Take the time you need to cry and be angry and accept what has happened. It does help to talk to someone--either someone you know or a professional. There is no need to feel as though you need to do it alone.
I've had three miscarriages over a period of 10 years or so...the last was just a year before I got pregnant with my now three year old daughter. It was also the hardest. I was 13 weeks and had JUST told my family the day before. I cried for days.
Ultrasound has a pretty big margin of error. I know you are distraught right now, but they can only give you the information that they have.
I never did have a D&C. My first two were quite early and were just heavy cramping and bleeding. The third was certainly worse...I essentially went into labor for 5-6 hours before it passed. I went through them all at home--there are risks--and got checked out the next day to ensure nothing was retained. I feel like I healed faster, mentally and physically, by letting nature take its course.
There are risks to waiting to let nature take its course...and and risks to going for the D&C. It is the waiting that will be the hardest if you go the natural route.
The hardest part of getting over it is the next pregnancy. My "safe" date with my daughter was the same date as my miscarriage the year before. I visited my midwife's office many more times than necessary to just hear the heartbeat one more time during the first 6 months of my pregnancy...then I was finally able to relax a bit.