Yikes! Using your extra fabric to fix is what I would recommend. The fabric is adorable by the way! You are close to the finish line here - pick a solution and move on. Since it is on three out of four of the corners my guess it is something you did that you usually do not do. Hopefully you will figure it out.
If it were me, I would rip out the three corners and put in new binding. Quick question - did you trim anything on the corners? I did that only once (lol) because I ended up with seams right at two corners, it looked too bulky, I was in a hurry and wasn't thinking. So snip snip ..... The resulting disaster looked similar. When I went to turn it, the cuts were right there. I ended up ripping out about a foot on each side and replacing the corners. Good luck!