Hello and welcome.
I know this second last paragraph of questions. Oh, I know it well. I asked myself all of those. I will tell you my conclusion, but this isn't what all the others found for themselves, therefore I am afraid, that a solid answer to those questions relevant for you can only be given by yourself.
There is no norm, just the way you are. Some seem to be 'mono by nature', meaning faling in love with one person, when coming across a new love interest, falling out of love with the old person and falling in love with the new one. But some are just like you discovered you are. If this is given 'by nature' or just an urge ... decide for yourself, there is no proven answer to that. But you should be able to answer this question: Can you continue living with your feelings without acting on them? And yes, acting on them will be selfish, as you do this for yourself. But this doesn't have to mean that you disregard the feelings of someone else. All I found I could do was be the person I simply am. I didn't want to live a lie or to disregard a part of me. As I knew that I wouldn't be able to ignore my feelings ultimately (I tried for years actually and it didn't work). Some people manage to do so, choosing to stay in a mono relationship while having poly feelings, but they hurt.
Our current situation is stable, I found a way through my personal inner jungle, but it took time. My husband and boyfriend had to do the same, but all of us felt relieved when things where out in the open, as none of us can live well with some hidden truth, the elephant in the room. First of all it is all about your decidion which road to take. Make up your mind when thinking about this long term. What will you be capable of and what do you want to be capable of when you thing about this?
Wishing you luck and some peace of mind.
Facts: 30, female, bi, v-type relationship with Sward (husband, straight, mono) and Lin (boyfriend, straight, mono), poly-fi and co-primary.