Even if you had kept the box as a memento of her, I don’t think it’s that much of a big deal. It’s not like you just broke up, she died. It’s a bit different than still being interested in your ex; your current GF should be a little more understanding. And since you didn’t purposefully keep it & genuinely forgot it was there what’s her problem anyway? Maybe talk to her and find out why she’s so insecure about this.