I have the most amazing, intense, crazy sex with a girl I've known for years. We dated a while back, but it didn't work out and now we just "catch up" to enjoy the great sex (when we're both single). Why can't sex with anyone else reach the same

Well , there is that indefinable ‘X’ factor that makes some people more compatible sexually than others. Whether it’s pheromones or similar tastes, or a combination of a lot of things, who knows? If it’s that good though, why not take a good long look at the two of you and see if you couldn’t make it work. It’s possible that you’re confusing expectations of sex and relationship and feeling they’re incompatible. For example, often people have fantastic sex when they live apart, and mediocre when they live together. They get complacent and stop making sex special. Maybe what you’ve done with your lover is try to have a ‘relationship’ based on what your expectations of what a relationship ‘should’ be, rather than what would work in the long term for the two of you.