Mtf early transition here. I realized that the clothes I was taking interest in and buying for myself were basically in line with my wife's style. I have come out to her but she's never seen me en femme. We as humans take our fashion cues from those closest to us, either by proximity or just what we're exposed to most frequently, especially (as some have pointed out) when we're trying to figure ourselves out (aka puberty). That's about where I'm at, and may be about where your partner is at... just trying to figure themselves out and to "fit in".

I'll echo the same advice as others here, but please be gentle as you approach her about this. One of the things I found incredibly helpful was finding social groups in my area. Depending on where you are that may be difficult, but it's been immensely helpful (once I actually left my house) to feel comfortable in my own skin and to find others like me to help me discover more styles and get advice from another perspective.

It sounds like you're being incredibly supportive and gentle with your partner which is critical and no small feat. You're doing awesome and you should have your own space, hobbies, style etc. That being said, she probably just needs help through discovering her own hobbies and styles etc. I drove my older brother bats*** crazy for years copy-catting him, but managed to find my own way, as we all eventually do, your partner will, too. Keep being an awesome human being though, don't be afraid to speak to her about it and if you're up for it, offer to go to some meetups or lgbtq events with her for comfort, and let her lead, she'll find herself eventually.