It is certainly possible to have romantic feelings (and to even be in love with) two people at the same time, although I wouldn't recommend it. As much as you may be attracted to the girl from your past, she has rejected you. Understand that and move on. Why would you want someone who makes you feel sad and acts as if she doesn't care? It's game-playing, insensitive to your feelings and a waste of time and emotional energy. Either she has lots of emotional baggage that you'd better be prepared to help her carry or she's not as into you as you are to her. Leave her in your past. I wonder if you're attracted to the thrill of the chase to some degree?
Be authentic about what you want. You're currently talking to someone who genuinely understands you and seems to value you. That's what healthy relationships look like. Is that what you want? If you don't feel the spark, with the new girl cut things off and remain just friends, but don't go back to someone who makes you feel glum.