What makes you think that you are stretching the fabric? I've been quilting for about 5 or 6 years, and making the same mistakes over and over, with the same poor results. (What is the definition of insanity?) This past year I have learned more than all the rest of the years put together. I was measuring wrong, cutting wrong, pinning wrong, stitching wrong, ironing wrong. I did nothing right. This year I leaned how to cut, how to measure, how to pin, how to sew, how to iron, and guess what, the results are amazing. Watch videos on measuring and cutting, get yourself fa presser foot with a seam guide on it, get a perfect square ruler by June Tailor. don't push your iron, just press it down, use a sharp blade in your rotary cutter, and a sharp needled in your machine. Get some fine pins that don't distort your fabric, and press those seams in the opposite directions.
Squaring your square or block is very important, it is not enough to just make the block square, you have to make sure it is squared to the center also. the June Tailor perfect half square triangle ruler, will help with that. Good luck to you.