I'm sorry, but I'm not quite following what you have and what you're doing. Are you making a pillow case (for sleeping) or a decorative pillow cover?

What do you mean by border? What kind of trim is it? Do you have a pic of what you're wanting to make?

You give 2 main fabric measurements. So is the "border" a different fabric to attach to the main fabric to make up the difference in size? Should the trim go all the way around or just on one side? Not following the trim size you're giving.

If you add all the pieces together, your pattern calls for 43 3/4" x 43" of fabric. If I read your statement correctly, you have a piece of fabric that is 10" shorter than what is needed. You'll need to add 10" somewhere, probably to mostly the border (I call that the cuff) instead of the trim. You really need more of the main fabric to help with the overall balanced look of the pillowcase. To answer your math question, here's a possible solution with a very wide trim and cuff piece:

Main Fabric: 21x43

Border (Cuff): 13 x 43

Trim: 9 3/4 x 43

Whatever you decide, as long as the three pieces add up to 43 3/4 x 43, you can sew the pillowcase. Hope this helps. Let us know what you decide or upload a photo of your finished project Happy Sewing! :)

A pillowcase too small to hold a standard pillow won't be usable. Since your main piece is too small, I suggest starting with a new main fabric, large enough so the directions work.

If you could post a link to the instructions, it might be helpful.