Has your vet suggested Zylkene? It was the first thing that my vet offered us for my high strung cat, and it definitely helps her to be much more relaxed! It's just a supplement that's made from something extracted from milk which has been shown to have a calming affect in small mammals (and yes, iirc they are currently testing to see if the same applies to humans lol).

We tried actual anxiety medication and that did help slightly more than Zylkene, but unfortunately my cat had the urine retention side affect so we stopped after a month.