Couple things - you did great! Part of the problem is that you have worked on it and worked on it and worked on it and you are getting frustrated. Step back, take a breath and then go back. You are probably going to be the only one who notices it unless you point it out. Also, if you use a high loft batting, it will almost disappear with the quilting. If you do your quilting in a brown thread with a wavy line down the sashing (fairly easy for a beginner) the eye will be drawn to it and I will almost guarantee no one will see the VERY SLIGHT tilt. And, if you find a quilter who says they never goofed - well, you just won't!