DH, I wasn't around when you were asking those questions before, but I don't understand the problem. Nobody is talking about completely and totally losing interest in an older partner, or wanting to leave the older partner--we're just talking about what it feels like to juggle new sexual desire and a more long-time partner.
It's completely normal for our brains to dwell on the new sex partner for awhile. And yes, I fantasize about all sorts of things while having sex with anyone. That's allowed! I assume my partners are doing the same thing... your brain is a free-for-all during sex. You can't control your thoughts, and why would you want to when they get you off?
You seem to have had a previous partner who pulled away from you in favour of your new partner. Sorry to hear that. The OP is looking for ways to *avoid* doing that, so she's on the right track!