I found Theanine makes me a little bit sleepy tbh. I use Phenibut and love the stuff, I've been reading Russian research on it and found out that it's not just a GABA B agonist but also contains PEA which explains why I find it mildly stimulating, motivating and that it gives me energy whereas Picamilon (also GABA B) I found rubbish and sedating.
I find all GABA A agonists I've tried so far sedating too and a lot of Theanine's actions on GABA will be on GABA A. I have thought it may be differences between GABA A and GABA B receptors before now so avoided GABA A to stop any sedation (because I still want to be able to function and work), but I'm now thinking GABAergics might all pretty much be sedating without a stimulant. Might be a good experiment to take a GABA A agonist with something stimulating I think.
As for serotonin, I thought that was supposed to make your mood better? I think I read somewhere that GABA has a negative effect on it though which might explain the cold feeling you describe. When I started Phenibut I was more social with people I wanted to be, but the remarkable effect was when I was out on the streets and not really after conversing with people I'd be completely switched off and immune to the world but still aware of it whilst usually my social anxiety would make me overthink and worry about people, I found it an amazing feeling tbh. Weird thing is I noticed I got more attention, but it seemed positive for a change, it could have been due to my behaviour and the way I held myself when Phenibut had its magic few months.
So I wouldn't discount Theanine just yet, I think it just needs combining with something stimulating. GABAergics aren't a silver bullet, they're a temporary plaster or a mask if you like, but they buy us time to figure ourselves out and find the really underlying problems and address them.