I suppose most people do recover from their lost first loves eventually and move on. I don't know that that particular magic, the enchantment of first love, is ever quite replicated in later relationships though, and as far as Chelsy and Harry are concerned, I get the feeling that both are quite impatient and restless people. There may be a lot of nostalgia when each looks back, and thus they might have found later partners wanting in some way. I don't know.
In an earlier post I did say that probably if Harry had not been who he was and is, they might well have been married (to each other) in their mid-twenties. In spite of the tempestuous nature of their relationship and their careers pulling them in different directions, there was a really strong bond and great love there. It's quite sad, really.
However, even if feelings were rekindled now, there is still that huge problem in that Chelsy doesn't want to be a Royal, and has never wanted that life. Even though Harry is only a part-time Royal still, there's the distinct possibility that he'll be needed in the BRF fold when Charles becomes King. I don't know that Chelsy's desire for freedom and disinterest in Royal engagements would ever overcome that hurdle,