I'm not sure how you mean you have a history of serotonin syndrome? I mean serotonin syndrome is caused by a bad combination of drugs, so you mean this has happened several times to you?

Yes, I guess the biggest problem with panic attacks is once you've had one, after that you suddenly have the fear of getting a panic attack, which itself triggers panic attacks.

Wouldn't worry too much about stevia and erythritol, though you shouldn't consume it excessively (apparently 8mg/kg per day is the upper bound for stevia consumption).

Perhaps this could have something to do with the keto diet though. It's kind of designed to mimic a starvation state, which could obviously be causing extra physiological stress. I know a lot of people rave about keto (and I'm sure it's great for many people), but a lot of people may also have trouble adapting to such an extreme diet. It's not necessarily a one-size-fits-all thing, so if you feel like it's causing you any continuous negative symptoms, you should consider going off it and not feel like you have 'failed' or whatever. Some people's metabolisms just don't work with it very well. Or you may want to try some kind of intermittent fasting, which can have similar benefits.

It also occurs to me just to mention that you are trying to do an awful lot of things at once here... sometimes in the nootropics community we get in this trap of constantly trying to add more and more supplements and practices and diets and whatever-the-fuck-else, but never once consider the possible benefits of scaling back and trying something simpler. I've never had a full blown panic attack but I have had some symptoms that were worryingly like one, anxiety for seemingly no reason, restlessness, etc. I worried about it a lot and tried a bunch of different stuff, and it took me the longest time to realize that just running a couple miles or doing some other intense exercise pretty much nips it in the bud.