Sometimes what happens, socially with older blogs is that they get picked up by a media writer, think Yahoo columns. They'll credit you and link back but won't even ask you. Maybe someone found it and posted on Reddit or tumblr.

Because you have a picture of Gandhi and the title itself is kind of catchy, I wouldn't be surprised someone else shared it on Reddit or even tumblr where the reposts are SO easy, even if they don't get read people repost for someone else...
If you have Googles Blogger you should be able to trace it down. Maybe conduct a G search based on date with that title... Unless you have analytics it would be hard to trace exactly. Maybe there is a software not even I am aware of.

The question I would ask myself is instead is:
Why did people share it. For that you don't have to trace so back. And the answers you get are way more valuable than you finding out who shared it first.
#Unthink

I use an app called Mention. It doesn't immediately show who made the original post, but you can find this data as you'll see a stream of all the mentions in date order.

I would focus all my energy into finding out what did I do correctly with the article and it resulted in people sharing it! In this way you can write more articles after this one and keep your self up and running. If you do so you can start your own blog and then even through a simple wordpress you can find out who shared in which country and when! So try to keep up the good work. All you need is making a daily programme and routine and have someone examining this article that you wrote in order to give you tips regarding the success of it.