I don't know if this is right but it's what I would try. Work the stitches back by taken your left needle into the last stitch and working it off the right needle back onto the left needle ,repeat to the end of cast on. Or start undoing the stitches by ripping back two rows and pick up your stitches and start again.
Mistakes are how I learn to not do that again.. Froggin has been my best teacher for teaching me how to replace my stitches right.
Someone will come and help , if this does n't help you.