My gacha seems cursed too. I got mainly old 6* or dupes. Old dupes. Same goes for the memory gacha.

Golden Week usually comes with a reroll gacha for 100 FG. It's a 11 roll gacha with one guaranteed random 6*.
Roll the gacha, pay 100 FG too keep the girls or reroll it as much as you want until you decide to keep and pay. It's an easy way to chase specific girls (if they are in the gacha) or get a lot of rainbow in one roll (if you're lucky).
Expect hundreds of rolls and several days to get something decent. And remember: don't be greedy.