First off, congrats on the great anatomy scan! Now you can relax and realize that your baby is doing just fine!
Heartburn is an extremely common pregnancy symptom. I never really had it until I was pregnant...then I started drinking Gaviscon like it was going out of style because my heartburn and acid reflux were SO BAD. I used to cry at night because of how much it hurt.
I am glad to hear that the zoloft has taken the edge off and that you are not as anxious anymore. Going up to 25 mg soon is a good idea. You definitely want to get to a therapeutic dose so that you can keep the anxiety away.
As for your sleep...I want to say don't stress it but I know it's hard. I know that when my anxiety is bad I have trouble sleeping. When my anxiety was horrible a few months ago I almost got no sleep at night, which then caused more anxiety. The best thing I did for myself was go on the zoloft and let it do it's thing. Once my anxiety was better my sleep returned to normal. I am sure that yours will too, but you just have to relax and let it happen.