Have you considered trying this?
Practice sewing them together using several of the cut triangles which you have NOT already sewn together and ripped out.
1. Mark your seam allowance on (the wrong side of) both pieces of 3-4 pairs of triangles.
2. Sew a pair together noticing where the intersections of the seam lines meet and making sure they meet on both pieces of the pair.....you'll have to turn it over to check.
3. After you've done this a couple of times, just mark the intersection of the seam lines with a dot, and try sewing 2-3 pair again. Are you still matching the dots on both sides of the pair?
4. If you are matching the sides properly, you can now likely eyeball where this seam allowance intersection is, notice how much point of each triangle hangs over, and can sew the rest of them together without much trouble.
Best luck!
Jan in VA