I have been going through a similar thing. I am not sure it is NRE or the separation that is causing the intense feelings while apart. Because of distance and work I can only see my boyfriends once a week at most sometimes it has been 2 to 3 weeks. We talk daily on the phone and texts. We didn't start poly but feelings grew. When I am with my husband I feel so close to him I am not missing my boyfriend as much, and Husband and I do talk about BF. It Brings us closer in many ways. He also allows me the freedom to text with BF at night while we are watching TV. It is when I am alone during the day that it is hard, almost painful, wondering when I will see him again. Then the extreme excitement when i do know I am going to see him soon and that is all i can think about. Then after we a date the dual emotions of elation of having been with him and disappointment of knowing I won't see him for a while. Pulling myself away from him is difficult. Yes addiction would be a very good word to use.
But on the flip side I do think about HB when I am with my BF. I have had extended time (several days) with my BF and i did miss my HB and think about him during that time is so of the same ways. It was easier to leave HB because I knew and end time and when I would see him and I get SO much more time with him. But I do miss him and think about him when we are apart also. Just makes me think it is more the separation than NRE. Or maybe the separation is extending the NRE since boyfriend and I have been together for almost a year.