When I go to google and search [costumemachine.com] your site is the first result and it has six sitelinks.
When you are looking at your metrics for the last year and comparing it to times prior, what is it that you see has changed? Are you doing OK in organic Bing and Yahoo and just having problems with Google?
That said, I'm having a very hard time finding 15 pages of original content, much less 1,500 pages of original content. Like others have mentioned, the tags and filters seem to be off. When I'm looking at 1st Birthday Costumes, the list of other 1st birthday costumes includes a hot dog adult link, as well as links that have no results (covenant slayer faith). Correction: now that I look more carefully at the page, it says nothing found for 1st birthday party theme, even though I clicked a link there, so it's just showing me random stuff for adults.
Like the others mentioned, I'm having a hard time seeing the value of this site to a user. Google wants people to have a good experience when a user finds a site, and there are other sites in this area that do a better job.