The way i see LOTR elves, is that they are very much stagnant.
The birth and death rates are almost zero except during times of war for the later. Their communities have barely expanded over the millenia, and there are a good number who can count their ages with very high four digit numbers. This could be a biology thing, perhaps menopause sets in earlier, or they have longer cycles; and perhaps its more of a mental thing, ever lasting life having caused them to loose a lot of their sense of passion.
They're very self contained, the elves of Lorien pretty much don't allow outsiders into their city.
They're not very inventive, or have no inventive desire. Despite having millenia to develop their skills and to discover new technologies, the elf technological level seems to have stayed completely static from the time of the armies marching on mordor and killing Sauron, to the time told in LOTR. They've invented nothing, they've achieved nothing.
All they've really done, is eat, sing and chill out for thousands of years.
I personnally don't think you can simply just exist for so long doing nothing, staying in the same area for so long. Sooner or later it will impact the mind, no matter how spiritual you are.
My concept of Memereth was someone who has existed for so very long, someone who could make predicitions of what is to come. She knows how to read palms, the cards and to read the portents, and with the right drugs, to see more ... though she could just be hallucinating
. Over all, life has long lost its element of surprise for her, and in an effort to break out of the cycle of unchanging existance and to stop herself from going mad; she reinvents herself every hundred years, sets new goals for herself. And if there is one thing she really likes to do .... it's to defy fate.
Given tolkien's stories, i'd have to say the majority of elves have simply gotten into a routine of existing, they're very dull, for example ... all of lorien. But there are a few elves, who grasp onto life so much more, and perhaps their time with the mortals helps them to do it. Those being elves like legolas. I do think that there is perhaps an almost vitalising effect the mortals have on elves as the mortals live with so much more urgency.
and then again i could be talking crap