you said you sewed close to your diagonal line- then cut---that is where you went wrong- you are supposed to sew 1/4" away from the diagonal line on each side of it- then cut on the line- so you have 1/4" seam allowance on each piece-
if the fabric is not too frayed you could first iron the quilt- getting everything lined up as close as possible- nice & neat-flat- then either using a zig-zag or other decorative stitch sew each one back down- making sure your stitch is wide enough to securily attach both sides... consider it a 'design element' you could use a contrasting thread- or a veregated thread and it would look as if you made it that way on purpose. l