I try never to deal with 1.5" squares. If I need a 1.5" sashing block, I sew a 1.5" strip to the sashing fabric and then I
sub-cut that to make a sashing with the sashing block attached.
However, that doesn't help if you have already cut all your 1.5" squares. One possibility is to buy 1/4" wonder-under
or steam-a-seam lite and use that to attach your squares to your sashes.
I am sorry that you are feeling so unhappy with quilting but I find that problems such as yours are most likely to arise
when sewing under the pressure of a deadline.
GOOD LUCK!
JUDY