i have always been a naturally observant person. i believe i inherited it from my father. even before i became more "aware" or "safety focused", i always watched people and listened in on their conversations without even thinking about it. in fact, it is difficult for me to refrain from doing this even when i try. so situational awareness was a bit easier for me to put into practice than perhaps some others. though it can be tiring at times, and i will agree you cannot live in "condition yellow" all the time, you really ought to be able to maintain SA in these situations without undue stress.
is it perhaps possible that what you are considering yellow is really closer to red? might you be exaggerating the threat? just asking...no offense intended.
i too have reached a point where i tend to avoid places with lots of people i don't know. however, when placed in such situations, i find that a relatively quick scan of the crowd every once in awhile will allow you to easily eliminate a fair portion of the folks as potential threats. with some practice, you can begin to decide who does not warrant further attention. you can also, with practice, identify the folks that you may need to keep a bit more of an eye on.
maybe this is all blatantly obvious...if so, my apologies. but i find that i can maintain SA in these situations by limiting the number of people in my area that i really need to pay any mind to.
or maybe i'm just not doing it right. or maybe i find it less stressful because i put myself in those situations so rarely.
YMMV