Welcome to the world of quilting. We all have made many many mistakes and still do but learn to live with them and learn from them. I always make sure each square is true to it's size before I join them, that helps a bunch. I always measure each side before I put on the borders. If you measured the squares they should be really close, if they are off a little I make the sashing the same for each side and make the fabric fit and I do that for each border, if you just sew the border on it becomes wavy. I mark the center of each border and each side I'm putting it on and pin them together and then pin the sides to the border. And even after this sometimes GRRRRRR it still isn't quite right so I give up and decide wavy ain't all that bad. Your quilt is wonderful, you should be very proud of yourself and no one other than you is going to notice any imperfections it may have just don't tell them like I do.