I've been taking 400mg of Magnesium Glycinate two times a day along with 200mg of Theanine (also twice a day) with my usual two cups of coffee. I feel fantastically focused without the jittery edge that accompanies caffeine alone.
You might find this interesting wrt magnesium supplementation: https://www.gwern.net/Nootropics#magnesium Beyond that, I wouldn't be surprised if the glycine along with the magnesium had neuro-inhibitory effect. Supplementation with gelatin might be worth trying for you. https://www.google.com/search?q=glycine%20in%20gelatin I put collagen hydrolysate in my coffee and I think it's pretty great.
Why is that?