I personally think it’s not a good idea for girls to wear tefillin. yes maybe some righteous women in the past were able to do it, like Rashi’s daughters, but this generation is not on their level of kedusha.
Some girls feel that it enhances their prayers. While that may be, you need to weight the pro and con. There are a lot of conditions that need to be met. If we can do it perfectly, that’s good. But if we mess up, then it becomes an aveira instead of a mitzvah. Why, because it’s Not our obligation to do it. Men are obligated to do it, and so if they mess up, it’s still a mitzvah because they were told to do it.
What people need to understand is that Hashem knows what people need. And so if men are told to wear tefillin and women are not, then it’s because we don’t spiritually need it. We should focus on the mitzvos that Hashem wants us to do. 🙂