Now when zooming out it is like this:
It is clearly visible that the extend part wasn't part of the picture. Do you have any clue how I can fix this? I'm just trying to let the image look like it has never been extended, so remove of the line and such in the middle. How can I do this in Photoshop? I'm not sure what terms to google for.
Filter > Vanishing Point...
. You draw in the perspective grid(s) and then either paste in images or use the clone stamp tool.