I had the same problem when I got married (all those many years ago). I'd practised my new signature over and over again, making sure that there wasn't too much of a "flourish", yet on the first occasion I had to sign something I signed it with my maiden name.
Is this a mental block? Are you having the same problem? I think it's going to be another week or so before it finally sinks in.
Photo from "Optical Illusion" on Flickr