What to do if my mother is the Trustee of money that my great-great aunt left me when she passed away but now my mother is keeping it from me?

I am so sorry for your loss. A Trustee is a type of fiduciary which is a person entrusted with great power as well as great responsibilities under the law.  If  person is not acting in the best interest of the trust or the beneficiaries of the trust then they could be said to be in "breach" of their duties which is a very serious thing. The terms of the trust (what it says) as well as the law in your state will decide if your Mom is acting properly.  Please take a copy to an attorney in your area to review.  Good luck.