It's helped me understand comparisons by having two lovers...
I love them both, and they are very different.
One is not better than the other, they are just different. Male/Female, Muscular/Curvy, Punny/Snarky, Each person is a wonderful package... yet very different. (and yes, there are similarities.. intelligence, acceptance, energy levels)
He's not dating her because he wants you to turn into her... but because she is different. And their relationship is different.
RE: Stopping a thought... when I've realized I'm focused on something negative, I'll start wearing a rubber band on my wrist. Then when I think about the negative (i.e. work during the weekend, or an ex-boyfriend's latest troll) I'll snap the rubber band. It gets my attention, focuses me on the pain of the NOW...