do not ... i repeat ... do NOT sew directly on top of the line you draw from corner to corner. instead, sew about 1/16th of an inch inside (to the right) of the line. that's about 2 threads. (this will work if you sew farther inside, too. the point is just to not sew right on top of that line.)
press the top square back ... carefully ... to form the new square. don't trim anything until you have your square.
what many of us do is start with squares that are just a wee bit too big (1/8th" 1/4" larger). once the lines are sewn and the square pressed open, trim it back to the size you want. it's a bit more work and a tad more fabric, but a WHOLE lot less stressful.