"PS, I do have a history with mercury- I got really sick after a dental procedure in ~2000, where I swallowed a little chunk of an Amalgam filling. "
Uh OH! You might want to consider the very real possibility that the chunk of mercury filling is lodged somewhere in your intestines. If that were the case, then ANY chelator you took would make you sicker, not better, because it would draw it right off the filling and into your bloodstream.
Have you colon cleansed? I highly recommend a very thorough colon cleansing protocol... several different ones in fact, until you are ABSOLUTELY CERTAIN that your intestines are 100% free of ALL old debris that could be holding onto that filling chunk. Definitely use products that soften old crusty fecal impaction as well as have Bentonite clay which draws out toxins. There is a photo I will never forget: of a chronic fibromyalgia sufferer with 2 silver fillings lodged right in her appendix. She had to get it removed. So...it can and does happen!
Also highly consider that you have intestinal candida in the fungal form from your mercury exposure. It is also possible that one of these colonies have grown around or even enveloped this filling particle... if this were the case you would be in big trouble because candida readily methylates mercury (which then gets absorbed into your bloodstream, later converting back to inorganic mercury which is what is really toxic).
As for you chelation protocol, you are on the right track. If it is not making you feel better, it is not working. If it is making you worse, STOP! DMSA and ALA are inexpensive and you can do effective chelation for roughly 50 bucks a month.