Seams can take up an amazing amount of yardage, you probably had enough for full sized pieces.
You can try to match the missing yardage, even if it is years old some of us with large stashes may have it.
Or, I know you had a vision for this project but maybe you can use it as a design opportunity to piece that upper corner, maybe going beyond the missing amount and/or doing something similar in the opposite corner so you can use that amount of background fabric to tie stuff in. I didn't immediately get the image of a shell, just a modern shape and colors until I saw the name of the picture. Some abstract-y beach or water in the corners would work lovely. Don't give up quite yet! I say that understanding the devastation to the heart when things don't come out true to the mental vision. For the rest of us, it's still a lovely project.
It's one of the reasons I like rehoming quilts (even if intended for me) as opposed to keeping them. I always know the secrets/mistakes that the recipients have no idea!