After I lost one of the twins, I live in constant dread that I'll lose the other one.

Scarlett • Hello there. My name is Scarlett. I'm 24 years old, married, and expecting my first in March.
Unfortunately, I can't get excited about my pregnancy because I'm trying to mentally prepare myself for the worst. Pregnancies are fickle. I heard and read so many stories of very healthy pregnancies, then poof...baby disappears. Truly, it doesn't seem to matter what you do - it just passes away.
I'm in constant fear that I'll lose this one. I'm 13 weeks and I get an ultrasound every week because I'm high-risk. The ultrasounds every week provide me comfort in knowing it's still alive. 
I don't know of any way to cope with this other to expect the worse, so my reaction wouldn't be as severe. However, when I discovered I had twins at a 6 week transvaginal ultrasound, I prepared for an inevitable miscarriage. I took it harder than I expected. 
Everyone tells me not to worry and relax - I'm prone to being doom and gloom about things. I have an anxiety disorder and it's not helping my situation at all. 
Anyone ever feel this way? How do you manage it?