I'm not an expert, but from everything I've read, the majority of ovarian cysts (even complex ones) are benign. But I totally get the worry about having a complex cyst - I was hugely worried about mine for a few weeks after I found out about it.
Thankfully, I've gotten less anxious about it as time's gone by. It helps that I've been really busy lately so I've had less time to fixate on it.
Of course, having said that, I still don't know whether my complex ovarian cyst is still there, or whether it's gotten larger/smaller, etc - I'll be finding that out in a few weeks when I have my repeat ultrasound.
I'm guessing in your case (as with mine), they're assuming it will have resolved on its own (ie: gone away) and just want to follow-up to confirm this.
And of course, the waiting is the hardest part - thankfully, your follow-up is almost here so you'll have more concrete answers soon.
Sorry I don't have any expert advice - but I'm sending good thoughts your way.
Hang in there!