Tumblr Custom Domain & Google

I know this topic has been addressed here before, but I'm looking for a bit more info. I just moved my Tumblr blog to a custom domain. The Tumblr url got heavy traffic, largely due to high Google rankings. Now I've fallen completely out of the rankings. My new domain is indexed, & I've verified it at Google Webmaster Tools, but I'd hate to lose my high rankings forever. Does anyone have any idea how long I can expect to see my rankings suffer? Would going back to my Tumblr url make any difference, or is that indexing already gone? Thanks!