This is how I do it. If the border is longer than the width of fabric, I cut two strips, piece them diagonally, and THEN measure the proper length... works every time!! And then you have a little leeway on where to put the seam, toward the corner or toward the center...

I don't understand what sewing the borders on the diagonal means. Do you mean you cut the borders on the bias? I don't understand what you mean by seaming diagonally. In any case, I'm sorry it's giving you grief!

Sometimes ya just gotta blow off steam! and we're here to support and listen!

I can not tell you how many projects I've had to put down and walk away from for awhile. I went and mowed the yard last time. A little physical release does wonders. I tell people THAT'S why I have so many projects going at once, so when one frustrates me, I can work on another one. Yeah, that's why.

Sorry it is getting the best of you at this time. You will overcome it, I know you will. I will just come to you.
Have a better day tomorrow, blessing on you. From someone who makes things harder than they need to be,
keep on sewing.