You might consider pulling back from the the new relationship (and dating in general) while you work through the whether and how of ending things with your long term partner. If you think NRE might be clouding your judgment, step away from it.
Of course, to some degree, it's difficult for anyone to maintain a degree of romance in a long term relationship. Sparks fade, and the slow burn of a long term relationship energy feels different.
But if there's no heat, that can be grounds to change. But give yourself a little room to focus on this decision, without the wild rush on NRE distracting you.