Caley, sewing triangles is a pain, so there's a "rule of thumb" that if you want, say a 4 1/2" hst, the rule is that you cut your 2 contrasting squares at 4 1/2" plus 7/8" inch, then draw 2 diagonal lines a 1/4" from the corner, sew on those lines, thus you're not having to mess with the bias, then cut between the sew lines, press, square to the 4 1/2". Personally, though I give it an inch instead of the 7/8". If you have to cut/sew on the bias, starch the snot out of the fabric before you cut. If you have any questions, feel free to PM me.
But really, your pinwheels look very good. Don't be so hard on yourself.