A small heart is harder to do then a big heart. Use a small stitch length and I leave the opening for turning on the straight edge. If you have a pinking blade or pinking shears, doing that all around the edge before turning will help. If you can't pink the edges then make little clips at intervals all around the edges almost to the stitching line. Make sure to do a clip directly into the V of the heart.
By the way, it is easier to draw and sew the heart shape on 2 squares of fabric and trim the seam allowance after the stitching.