It seems to me it could be either way from what you are saying, but I'm leaning towards "you are poly and just getting used to dealing with it".
The NRE can make you feel like the ORE is tame in comparison. But the Old Relationship Energy, if you have it, is something to take into account. The way you know each other, understand each other, feel like he's part of your life.
Can you sincerely imagine breaking up with him without the idea making you sad, and feeling like this would be a mistake?
Your relationship with your DH doesn't have to be the same as it used to be. I would even say, there is NO WAY for it to stay the same as it used to be. You are in a different stage now. That doesn't mean it is over. That means the previous stage is over. The difference between your NRE and your ORE might make you feel like your first relationship is dying, but I think it probably isn't the case.
Having a boyfriend and NRE with him, while having a husband as well, I can definitely understand why so many people leave a relationship for the newer one. Not that I would ever do it myself, but the NRE can definitely cloud your judgement. Everything you feel or do for the new person makes you focus on the fact you don't feel or do it for the other one. Or not in the same way. Or you remember how it used to be with your husband but isn't anymore.
But that's normal. Relationships change and evolve, but it is still love, even if it takes a different shape. I think it's important not to break up with someone during your NRE, and wait until it's passed. THEN, you'll be able to tell if your relationship is over or not, but right now you're too likely to make a rash decision, I believe.
Note to Rag: no, I'm not having doubts about our relationship. But I have had a few times when I, too, went all "what if I'm not poly? What if it's just that this relationship isn't working out?" both when I didn't have anyone else in mind and after I had a boyfriend.
I think there is overwhelming evidence that I am in fact poly and that our relationship is working, but even I still had these doubts every so often, so I can totally relate to the OP. It's very easy to second guess yourself, especially when you've been raised to believe in the norm.
Actually I think Rag too experienced that. When he developed feelings for his interest, J, he first thought it was a sign that there was some problem with our relationship.