I bought Raft here on the website long before it came out on steam and now it's availible for free here. 

What happens to us who bought it? Do we just have to live with wasted money? Or is there a posibility to get a steam key?

Just to clarify. You did not buy it, you made voluntary donation to support the developers. Itch.io raft has always been free with option to donate. And no, those who donated wont get a key for Steam version.

“Will players who have played or donated on Itch.io receive a free copy of Raft on Steam?”
We very much appreciate all the support and feedback we have gotten from our community, however, playing or donating to the itch prototype will not grant access to the Steam version of Raft.