This is my first relationship, we'd been together for a year before she moved and been living together since the end of last year. I was aware of her wishes to be polyamorous going into the relationship, but it wasn't something that was brought up constantly, and I made the mistake of putting it in the "I'll worry about it when there's something to worry about" basket, feeling like it wasn't really something I could prepare for or get used to without an amount of exposure to it.
The reasons I'm feeling uncomfortable with it are because I perceive physical affection in the way that it holds true for me, for example, only having sex with someone I wholly love and trust and have gotten to know for a long time, and find it difficult to distance myself from how I feel about it and accept what it means for her. So far I've only been able to do that with kissing and cuddling. This then leads to me feeling not good enough because she's doing all these close things (from my perception) with another person. I like the idea of sharing herself and being happy in that way logically, I'm just not there emotionally yet.