Ultrasound and Beat anxiety

So I was determined not to panic the whole first trimester and that I was going to be positive and enjoy this pregnancy. And I lasted like 9 days! Then I had Beta 3 four days after beta 2. My number was 881 and the nurse who called me confidently said that my numbers were good and everything looked great and they'd see me for my ultrasound we had already scheduled for thursday. But then I got home and started calculating and looking at tables and charts and decided given I was at 375 at 15dpo, it should have been over 1000 not 881. So I panicked, worried I was miscarrying and finally called the dr office back in a tizzy.

I finally heard back like 5 hours later and the nurse again did not seem concerned but did humor me and got out a calculator and said if they had gone up 60% every 48 hours, I should have been a bit over 900. So it was a small amount under expected levels but nothing to be worried about. Number change all the time and it will probably surge again and be a lot higher in a few days. Happens all the time. Also she said it was possible I had a twin pregnancy and lost one of the two or one is going much slower than the other and that could explain it. Either way my levels are still high for 19dpo and did still go up a lot so apparently there is no reason to worry. I offered to come back in but they said not necessary, just come thursday, no reason to worry.

So I know given the dr told me not to worry, I shouldn't but of course I am worried. And I still have about 36 hours until the ultrasound- omg, it's killing me. And i'm worried about every symptom even though I don't doubt they are just pregnancy symptoms. Like why does my back hurt a bit- probably because of hormones or reaching up and stacking clothes at work all day. No bleeding and no cramping so no real reason to worry...

Anyway, must not go crazy and worry myself sick... easier said than done, of course! Anyway, if you're reading, please pray for a beautiful sac on Thursday morning!