It sounds like you have been used to a more casual version of poly until now. *Open* might be a good term for your relationship dynamic, but that is up to you really. Some people describe their poly as having many sexual experiences with where they kind of know the people and everyone knows that you are doing things with others.
This is different it sounds like. The possibility of something real to invest in. I can imagine that is really scary for you as the one watching it and being the same old same old. Possibility doesn't mean that everything ends though. Love doesn't mean everyrhing else ends either. What it means is that you will need to be clear about your needs and ensure they are met as much as your partner can. Most of the time people baring the brunt of nre need reassurance, extra words of compassion and understsanding, and time where the other person is not a part of the conversation or being text throughout.
Now that you are in it, it sounds like you are communicating and making sure that your partner knows you need extra attention. That's all you can really do. Going at the pace of the one that is struggling the most might help; that being you. There is no rush in all this, take your time to investigate how you feel. I don't suggest avoiding or ignoring your feelings. Embrace them and learn from them. Talk to people, read around here and get comfortable with the change. It sounds like you are doing fine. If you do a search in the tags here for nre, you will see that at least what you are going through is common.
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