Its unusual for a 20 year old to realize they are mercury poisoned. What are your symptoms for the record.
If DMSA doesnt cross the BBB then it wont directly affect brain mercury. It is too early in your situation to even tell if you need DMSA.
My recommendations is to take ALA,eat meat,take vitamins,do saunas for the first year. OVERCHELATING will cause mercury to move into your brain.
Also stop eating all fish,stop taking calcium and magnesium supplements as they are made from sea creatures. AVOID aluminum added to baking powder and aluminum pots and pans.
Take your basic vitamins.
See how you feel in six months.