Wow! I find this whole topic central to who I am; it is ingrained in every thought and action to a large extent. The process of dealing with one's insatiable idealism can be so exhausting and so rejuvenating, but in a cyclical way. I would agree that those moments where one discovers that their actions can have little impact, or worse, can corrupt or destroy, is incredibly humbling and a wave of "knowing" just rushes over me that I'm overwhelmed by the clarity.
I was in the Peace Corps and lived in rural Paraguay for two years with no running water, no electricity but that wasn't the difficulty for me: I began to realize that my presence there altered what was good and strong about the community...I saw a beginning dependence on me which frightened me. I altered my actions to minimize it, but my mere presence, I believe had a negative impact. Yes I taught programs in early childhood education, trained medics, taught about malnutrition, identified volunteers to carry on programs but I still felt I was breaking a very delicate fiber of the community. I also believe that there was a subtle pressure to identify leaders within the community that put their lives in danger considering it was still a dictatorship under Stroessner (side note: there was a coup while I was there). Keep in mind, I also experienced moments of "enlightenment" (sounds cheesy, I know) but there is no other description for the feelings and emotions that would wash over me as I looked down the valley while on my horse.
I gained more than I could ever "give" from the experience and am so enriched by what I experienced. I then also realized why go to another part of the world when our own society is in need? So teaching, and then advocacy were paths taken by me. However, I find myself constantly evaluating my impact on a person's life: I will never make decisions for a person (I work with people with mental illness, legal issues, etc.), I will respect their decisions because I am not them, I cannot possibly know everything they "feel." I know they are in pain, and I have to be careful not to soak up their pain and anxiety, but I will not profess to know what is best; I abhor that mentality.
My most recent idealistic foray involved a group action of dissent, arrest, trial and jail time. I was incredibly optimistic because I thought this time is going to be different (stupid me) but I was of course crushed by the jury's verdict and how our peaceful action was characterized. I was exhausted by the process and had to take a step back, which I am doing now, but I know it will end, and I will become energized and begin the fight anew.
So, dealing with my own insignificance in light of all that needs to be done is something I struggle with every day but I have to always look for balance. I know myself that by doing something, anything it somehow helps me deal with my own internal suffering when I see that the world is so broken and there is so much pain. If I did nothing, I think the pain would consume me. That, and I don't let myself read world news stories for hour after hour, I have to set limits.
Thank you for bringing this topic up; so nice to know others struggle with this as well.