I have the same issue with zinc. I had been Chelating Mercury with DMSA for a year of and on, and just kept getting worse, I figured it was because it was taking out my zinc so when i tried so supplement, i found it very difficult. Through trial and error i found that i could tolerate 12mg of zinc/day.
For the past year i havent been Chelating, just slowly ramping up my dose of zinc and vitamin c. I use chewable losenges because they have 23mg of zinc and some vit-c, so i could break them into small pieces and add zinc at about 3mg at a time. Still have a week or so of bad reactions after each ramp up. now i am up to 70mg/zinc and 230mg/vit-c per day. my motivation and energy has gone way up, and ive had a very productive year despite the ramping up side effects. Now i'm trying to remove copper and am having a very difficult time, seems like there's a lot in there! taking 1 to 5 miligrams/day of molybdenium for that. Thinking of going back on DMSA. Hope this helps. good luck.