Can my ex girlfriend legally keep my stuff?
It depends on where you live and whether you were living together.
I can only speak from my own personal experience, so not sure if this will help:
I divorced my ex-husband 7 years ago. While I was having a shower, he sneaked into the master bedroom and "confiscated" my wedding ring (as he had done numerous times when I was being a "bad wife"). This ring was worth quite a lot, and as I was trying to negotiate a fair financial settlement, he refused to return the ring (keep in mind, he had over $50k in savings, which I contributed to). Anyway, long story short, I asked my lawyer if I could report the ring as stolen to the police. Their advice was that because we were living together, it wasn't technically possible for it to be considered "stolen", so there was nothing much I could do.
In the end, I cut my losses and moved on. I was worse off financially, but ultimately got rid of a very toxic and controlling human being in my life. THAT was worth every penny.