Just noticed the blue hearts...Love them!
When doing the cornerstones like you did (and the blocks for that matter)... make sure the center squares are exactly the same size, and that they are the size you want the finished block. Cut the corner pieces as squares...most patterns are worked that way. Draw a line to exactly divide the corner squares in half diagonally...don't guess...draw the line. I use a pencil.
This line is your stitching line. Lay the corner square on the center square, matching the corners exactly...pin at the inside area. Sew, follow the line exactly.
You will get exact corners this way. If you don't want to waste fabric...before you trim...sew 1/2 inch out from the first stitching...draw a line, or use the presser foot...does not have to be exact. cut right between the two rows of stitching...you will have half square triangles pre sewn for another project, or you can use in this project...the border maybe?
If you do the above & make sure you don't sew over the areas that form points, you will soon be making these blocks without even thinking about them.