My first question is: How did you measure for the added borders? Did you measure the center and add that length of border? It appears that you have too much border and if both added borders were not measured at the center, then that is the culprit. It that's the case, I would take the quilt out of the frame, resize and reattach the borders and start quilting again. I know that's not what you want to hear. But if your borders were measured correctly, it must be a bias issue. Then, you're challenged with 'making it work'.
I just finished quilting a small throw that had some bias cuts in it and it was wavy all the way to the end. I just kept maneuvering the fabric until I got to the end. About midway, I had to "ease" it into place so I wouldn't have too much gathered at the end. I don't like bias cuts!
Good Luck with your finished product. I like the B&W contrast.