There's no way to tell a person how to stop worrying. I can however, share my story, maybe it will help (and maybe not). I have Multiple Sclerosis (MS). My first pregnancy was a text book pregnancy. Everything was great the whole way through. Had some issues with a lot of contractions at the end, that did go away, and was induced 10 days early.
My second pregancy started out text book and we thought everything was going to be great. At 6 months my MS came out of remission and I started having a lot of problems. My neurologist pulled me out of work and put me on limited activity (very weird for him as he doesn't condone laziness). Both my OB and Neuro said not to worry. At the same time, my OB ordered a stress test for 3 times a week (Monday, Wednesday & Friday). A few weeks into that I began receiving steroids for the MS. By the time I was at 36 weeks I had an amnio to determine if her lungs were strong enough (two courses of steroids later) and I was induced that afternoon. I was the only new mom who had an IV in her arm and couldn't really do much for myself - but we were all okay. Did I worry through it all, definitly. The nurses who took care of me at the High Risk practice I was sent to for stress tests were great. I asked every time if everything looked okay. The slightest contraction had me concerned. I worried about everything and didn't push myself to much at home (hard with an 18 month). Besides a little reflux issue, which she grew out of, she's fine today (3.5 years later).
You need to trust the doctors, they won't deliver your son early unless there is a valid reason or if he is ready to deliver (lungs are mature). In the mean time, even though they will tell you to worry - you will, it's just normal.