Hello and welcome!
As you already know since you've experienced this before, insomnia is one of the common side effects of Zoloft. Some people have that, and others feel more drowsy. I would recommed trying the Trazadone, it's a good medication for insomnia. Definitely a better choice than the Ambien for this kind of short term course. You should only need something to help you sleep until the insomnia side effects start improving, which they will. Typically, side effects start improving after about 2 weeks of therapy, give or take.
Also, if you happen to be taking the Zoloft at night, ask your doc about dosing in the morning instead. Changing dose times is often fix enough for these kinds of side effects.
Hang in there...it will get better! It's great that you recognized the need to seek out help. That's one great thing about having a history of anxiety, we become much more "in tune" with the way we're feeling and can quickly identify when our anxiety is becoming troublesome again. We're much more apt to seek out help and restart our treatment methods quicker after having been through it before!
Keep in touch!
I would give the Zoloft some more time. Maybe it's your body getting adjusted to the medicine. If after 6 wks you still experience insomnia, then you should talk to your doctor about taking a diff medicine. When I was on Zoloft I had an opposite effect - lethargic. Good luck!