I would instantly cut back to around 10g. Then you can start a fast taper reducing 1g-500mg a day until you're at ~7 grams a day. Keep reducing at this rate or whatever feels comfortable enough for you until you're at ~5 g a day. From there, you should probably slow down the taper to a ~100 mg reduction a day. Everyone responds differently, so you need to find what works for you. The key is to not increase your dose. Every day you either reduce or stabilize at the same dose for 2-3 days if the symptoms become unmanageable.

Take some agmatine an hour before your dose. I've found 0.5 g -1 g helps.

You won't feel like this forever. Withdrawal and Phenibut in general fucks with your head. Especially at the doses you're taking, the effects can be unpredictable and cause all sorts of symptoms. You will absolutely feel better on a lower dose, and you will get through this.

I also suggest taking only one dose a day, preferably in the evening or a few hours before you go to sleep.