I have been quilting for 30 years - still not perfect....lol...but, the girls are right. You must double check your cutting - must use a rotary cutter and be sure to line up your ruler markings the same way each time you cut (are you inside the heavy black lines or on the outside edge of them). But the number one thing I have found is in pressing! I use a cut & press board with ruler markings on it - and I actually block the quilt squares as I press with steam. Triangles especially will stretch...so blocking them to the correct size works wonders! Then, just keep doing your piecing...it really does improve with practice! Yours looks fine to me!