My girlfriend has previously taken place in poly relationships and had crushes on multiple people at once, but that was a few years ago and we’ve been monogamous since. However, now that I’m considering my own polyamory, she seems to consider being poly a sign of immaturity or lack of true love in a relationship? I don’t understand how she could have changed so drastically and it makes me feel ashamed of my feelings.

It sounds like she didn’t necessarily change, but that was her experience of polyamory all along. She was able to engage in poly relationships and multiple crushes because she saw it as less serious. This is actually pretty common - people ‘date around’ until they’re ready to ‘go steady’ with one person. (I would argue that’s not necessarily polyamory, just dating, but the past few generations put such a premium on monogamy even by elementary school that the concept of being someone who dates multiple people but expects to settle into a mono relationship is somehow less common, in my estimation.) Now that she’s gotten older and met someone she feels more deeply for, that phase in her life is over.

But while it’s totally fine that that’s her experience of polyamory, no one’s experience is universal. “I can’t do that” is not the same as “that’s impossible” and “this is how it feels for me” is a far cry from “this is how it is.” Unfortunately, very few people realize that, and most people extrapolate from their own experiences to generalize about what the world must be like, how feelings inherently work, what goes on in other people’s heads, etc.

There is nothing to be ashamed of - it’s fine to be poly, and it’s fine to be mono. Let your girlfriend know that while her dalliances with polyamory may have felt possible because of a lack of commitment or a casual, youthful experimentation, there are other ways to be poly. Maybe she can hear you, and is able to understand that your polyamory doesn’t mean you love her less or are less committed to her. But, maybe she won’t. It’s never possible to adjust someone else’s perspective through sheer force of will, and if she sees polyamory as inherently immature or requiring the absence of true love, that may or may not change.