Try potassium with your magnesium. Best to get it from food. try lots of fresh orange juice.
Normal adult needs 4700 MG a day. Orange juice is the highest potassium level I have found. I buy the version that has calcium and vitamin D added.
Lots of OJ. 8 oz has 400 mg of potassium. The supplements are OK as long as you are careful. I use both. Works immediately on BM.
Everything you will find on the web says DANGER when referring to potassium. Just the rhetoric of backwards thinking. I bought the DANGER thing and avoided supplementing for a long time. Natural sources like OJ are NOT dangerous and you get VIT C, A, D, Calcium and Potassium.
One word of caution: Stock your bathroom with reading material. You'll be there for awhile.