Petunia really got the short end of the genetic stick in her family, and Lily's distancing of herself as they grew older would just be the straw to break the camel's back.
In some ways it seems that finding in Severus someone else who also suffered from Lily's rejection she might find some solace, but it was never going to be enough when his heart lay elsewhere. I can't help feeling, though, that if it were all bad, then she wouldn't still be thinking of him years later.