That someone

When do you know someone likes you and wants to be with you, and when do you make the move to keep that person in your life? It's not that I was being too cautious and reserved, but rather showing her respect and concern by not making the move to a more serious relationship sooner. Now it's too late. She is gone from the city, and living somewhere else. The story of my life. Should I even continue to search for her, or the next her, or should I just continue onward and let fate take over? I am not a fatalist, but do think that some karma-ish intervention would suffice. Oh, my head in the clouds, keeping me awake in my dreams. Where do I step next?