i don't know why statecounter tell me that the keyword is still ranked on #1, and how did those two visitors got into my website
Google is serving personalised results. Perhaps your page was at #1 for these visitors.
If i continue doing backlinks..
Have you been building links since March? How many links have you built so far and from how many sites/domains?
...i still have some pages indexed, one of them is indexed on the second page...
I think you need to understand the difference between indexing and ranking and use the correct terminology, otherwise members of this forum will not understand properly what you are saying.
Page may be indexed, but may not rank. You can check whether a page is indexed by Google by (for example) copying a couple of unique sentences from that page and pasting it to Google search box. If you see your page appearing in results, the page is indexed.
However, this does not mean that this page will rank for any keywords you wish it to rank for. According to graphs you showed, Google has in its index 546 pages from your site, but according to the other graphs (query graphs), after the traffic drop only very few are ranking well enough to bring you the traffic.