I guess I'm defining poly as the orientation, rather than the habit; as in if you suddenly break up with your 3 girlfriends, you're still a polyamorous person, just single.
It feels similar to a sexual orientation to me. I'm with a man, but I'm still attracted to women because I'm bisexual. When I'm choosing to be monogamous, I'm still capable of loving many but am focusing my attention on one.
I thought that I was fairly ambivalent - happy with mono, happy with poly. Then I had a relationship with someone who tried to force monogamy on me and realised that it made me incredibly unsatisfied. Might have just been a shit boyfriend, though.
I feel like it depends really heavily on who I’m with. If my partner wants to be monogamous and I really care about them and I feel an incredibly deep connection with them then I can be monogamous with no issues whatsoever. The reverse is also true.