I've suffered from miserable heartburn for about 3 years. I took over the counter antacids and prescription acid blockers. All of them made my heartburn worse. My ND put me on a supplement called DGL Plus which contains Licorce Root, marshmallow root, slippery elm, and aloe vera. I take it twice a day 20 minutes before eating a meal. It didn't give me overnight relief, but after being on the supplement about 3-4 weeks, my heartburn symptoms are feeling tremendously better. My ND also suspects I have low stomach acid rather than too much (which explains why the acid blockers make me feel worse), so she's having me supplement my diet with a teaspoon of lemon juice mixed in warm water 15-30 minutes before every meal. This is actually a good test for anyone with heartburn to find out if you have too much or too little stomach acid. If lemon juice makes you feel worse, you have too much stomach acid, so you shouldn't take lemon juice. If you feel better, you have too little stomach acid.
I hope this helps people with persistent heartburn. I've had it really bad for a long time and I can't tell you how relieving it is not to have it 24/7.