How to undo: Tumblr import

I am completely new to wordpress blogging, and have made a silly mistake.
I imported my tumblr blog to my wordpress blog, not realizing that every single thing I reblogged on tumblr would show as a ‘new post’ in wordpress.

I have 2000+ posts now when I originally only had 4 ‘real’ posts.

I have tried deleting them by going to ‘screen options’ and displaying about 200 posts per page and using the ‘bulk action – move to trash’ function…. but an error keeps popping up.

All I want to do is ‘undo’ the silly tumblr mistake, and if thats not possible – do I really have to delete each post one by one? *sigh*

Any help would be greatly appreciated!!