If you trimmed the edges straight on your strips, this leaves all bias edges. Bias edges are going to stretch. I would carefully take apart the strips. When the strips are all separate, I would run a line of stay stitching (just shy of the seam allowance) down each side of the strip, coaxing the bias edges to come back together. I do this by nudging the bias edge before the needle towards it with my stiletto as I sew. Once the strips are all stay stitched I would put theirs back together alternating the direction in which each row is sewn.
They look like lovely French Braid strips and are well worth fixing. You could also add sashing as others have mentioned to stabilize the bias edges. Good Luck!