This old sweetheart could symbolize the "feminine within." According to some psychological theories, within each man is an inner woman who serves as a helper, guide, and source of feminine characteristics (compassion, empathy, etc.) It could be that your unconscious mind is concerned with your relationship with the inner feminine. To differentiate that from your current "outer" relationship, a different person is chosen as the symbol. I think it's common for women or girls we knew early in life to be chosen as symbols for the feminine within. I recently had a dream about a sweet girl I knew in grade school and haven't thought consciously about in 20 years.