Is going to be such a lovely quilt that I'd hate to see you give up on it. That said, the only way to fix the center is to take the sections apart and carefully put them together again. I'd do them two at a time, each quadrant if you will, lining those intersecting seams very carefully. Then two sections into halves, etc.
You've chosen lovely fabric, and it looks like everything is pretty good up until the sections were put together, so I'm sure you can fix it , irritating as taking all those seams out well be, and it will be stunning!