Oh, no, don't quit. In quilting there are always the three things you mentioned: cutting, sewing, pressing. To get it "right" (so that the finished block is the correct size) these all need to be accurate. First, try checking your seam allowance: cut 3 strips 3-1/2" x 1-1/2" and sew lengthwise with a quarter inch seam. Press to one side then measure. You should have a 3-1/2" square. If you don't, make adjustments by moving your needle or putting a piece of tape on your machine.