I do not have experience as a single parent. But I have experience as the child of a single parent, my mother having died when I was five. And the intermittent dating my father would attempt.
I think you should get to know the person you are dating well enough to establish that they would have a positive impact on your child's life before an introduction. If you do not feel comfortable introducing someone to your child they probably are not good for you either. As for whether it is a long term thing or not; loss hurts, but we need to learn to deal with it eventually. What is important is that the time together provides for positive growth. Keeping abuse and neglect out of the equation.