Meaning: what you don’t have always looks better than what you do have
Rewrite 1: the grass is always greener in your neighbor’s yard
Rewrite 2: the alternate route is always quicker
Rewrite 3: the other waiting line always goes faster
Rewrite 4: I knew I should have chosen the pecan pie instead of pumpkin!

Comment: There are probably no statistics to prove it, but we seem to always make the wrong choices. At the store, the cashier line you didn’t take always goes faster. At the drive-in bank, the line you choose is always slower. At the clothier, you knew you should have bought the green shirt instead of the blue one. No matter how well you water your lawn, the next door neighbor’s grass is always thicker and greener. We are so sure we could have made a better choice, we are never satisfied, so there are lots of choices for rewriting this cliché.