first relax! you can always undermine your foot. even a 1/4 inch foot. by that I mean you could be pushing your fabric toward the right of the foot or letting the feed dogs move the fabric away to the left. Either way, the seam wont' be straight.
I've seen quilters on t.v. let go of their piece before it's gone completely under the needle. that allows the fabric to move off kilter. so, keep your fingers on the fabric piece all the way through. you can follow the piece to the left of the foot with your hand, or if it's skinny, just up to the foot.
good luck. try again please. dont give up.