A lot of people say they are OK with something like poly and truly believe
it, then turn out not to be OK with it when it comes down to brass tacks.
I think that’s because it’s easier to convince oneself of something
intellectually than emotionally, especially if one hasn’t been in the
situation before. I could convince myself that I could jump out of a plane
with a parachute on my back, but when it came to actually doing it, I might
be more frightened than I expected.
It’s possible that your girlfriend said nasty things because she was hurting
a lot. I went through a similar situation — thinking I was OK with poly and
then turning out to feel very Not OK with a certain incident — and I said
and felt a lot of nasty things when I was very upset about it. Partly because
I felt my partner “should have known” that I wouldn’t like what happened (but
actually it was a difference in the way we thought of poly, not something
that he should automatically have known). Partly because I was angry with
myself at not keeping my word (saying I was OK with it and then not being OK
Something like this happens in many relationships that try to transition to
poly. Many relationships do survive it.
Slow down on enacting the poly stuff, but don’t hide or stuff your desire
for it. Keep talking. Allow a lot of time for the transition — months or
even years. If you really think your partner is someone you want to stay
with for a long time, it’s worth spending the time to do the transition
slowly and maintain the relationship.
Consider different ways of doing poly. Sleeping with someone your gf
doesn’t know, on a trip, can be stressful in ways that other forms of poly
(such as sharing someone, or starting a relationship with someone your gf
knows and likes) might not be.
Something that works for some people is to give the less comfortable person
a time-limited right of control over polyamorous activities: say something
like “I can’t promise to be monogamous for ever, but for now I will give
you control: you can tell me what you’re comfortable with.”
Many people respond to this by giving the control back (perhaps slowly over
a period of time). It is easier to say “yes” to someone, when you know
that you have the right to say yes and no, than it is to say “don’t” to
someone all the time, if zie is constantly pushing you to allow something
you don’t feel comfortable with.
Please continue talking to her and consider the possibility that she may
have genuinely believed that she was OK with poly, she may not have
expected her negative reaction, and when she said she kept the truth from
you, it might have been because she was upset.