@Blud serotonin can help, but if it's too high it will also kill libido. You can try 1g and then 2g tryptophan or even 100mg 5-htp to see if that helps.
Taurine, GABA, kratom and phenibut has helped me a lot.
If found that the things that help me the most are compounds that automatically relax my pelvic area. As if I'm automatically doing a reverse kegel.