Pregnant & Terrified

So... I am 21 years old and 6 weeks pregnant. Although I am extremely excited this pregnancy was unexpected and scares me to death. At age 16, I had a miscarriage at 18 weeks. It was the most traumatic, heartbreaking thing that has ever happened to me and I am absolutely terrified that it will happen again. During my last pregnancy at the beginning of my 18th week I began to bleeding heavily. I went to the dr. on a Friday and they told me that my cervix was a little shorter than it should be. They told me to go home and to come back after the weekend on Monday to see if anything had changed. After going home I began to have a lot of pain (not knowing I was having contractions), and ended up losing my daughter the next day. It was terrible and it absolutely terrifies me this time. I feel like I cannot even be excited about this pregnancy because that thought is just always in the back of my mind. I have my first dr. appt on the 23rd and I hope there is something I can do to prevent this from happening again. Any input or support would be much appreciated. Thank you!