The only way you're going to be able to do this is by placing a text box
over the AutoShape (instead of using Add Text to the AutoShape). Make the
text box large enough to cover the whole heart. Then center your text and
use line breaks to make the lines fit closely.
take a look at www.drawingwithword.com and find your way to the tutorial on
"Putting text inside a shape" which does what you want - with a slight
limitation