I have also tried to find the reason for my acid reflux/heartburn. I was diagnosed with IBS years ago but always had bouts of acid reflux since I was young. In recent years the feeling is mostly in the middle bottom of my ribs and even painful to touch when pressing against. I stopped eating chocolate, eating out, drinking alcohol, and i have been vegetarian most of my life. The hardest is coffee, now i can't even drink decaf as it makes it worse. I am also on continuous HRT, I tried vaginal Utrogestan but that caused bleeding and I read that sometimes it doesn't make difference regarding acid reflux.
I have been reading about possibly having low acid, tried using apple cider vinegar but it hasn't solved anything. One thing that helped for a while was Silicea gel. It really solved the problem but just for a while and it didn't make a big difference when I tried it second time. I would recommend it though.
After all of this, I have been on esomeprasole and then omeprazole for 5 weeks now. It has made it better but the pain is still here and occasional heartburn feeling. I have been referred for a blood test and a scan.
It is just so difficult to understand why this is happening when I have been so careful. It is so interesting to read this thread and I will try also some of the diets mentioned. I must admit that the Fodmap diet really helped me when I suffered from IBS, it is very restrictive and difficult when you prepare food for the family but maybe that's the way to go.