One thing I’d recommend before anything else is trying probiotics. If it works and you start feeling better, forget everything else relating to your diet. You could try yoghurt, yoghurt drinks, kefir, fermented products or a brand. (I’d recommend Bio-Kult, as it’s gentle and has multiple strains.) I’ve actually had similar problems to you, so I’m speaking from experience.
If you eat the same lunch or there is a common ingredient, stop eating it and see if symptoms improve. If you have shredded wheat for breakfast daily, have something else and see if the same thing happens a few hours later. It’s difficult figuring out what you react to, but when you get rid of it, life is so much better. You could also look up the low-FODMAP diet and try it for a couple of weeks.
For flatulence, you can purchase Shreddies. (I’m not joking.) It’s carbon-fibre underwear that is expensive but stops the smell of flatulence and anything, really. The anxiety is something I empathise with, but it likely makes things worse. The best times I have are when I forget about it. Things that reduce stress (yoga, meditation) on top of your regular exercise might help. You could also try peppermint or ginger to see if it soothes your intestines.
Another thing: it could just be IBS. It could be something else. This can happen out of nowhere, and it frequently does if there’s a history of digestive problems in the family. Once GPs say you have something, they don’t care anymore unless you come with new problems. Do as much as you can by yourself, and if that doesn’t work, ask for medicines from your GP.
Edit: You can also PM me for any extra advice, as I’ve been dealing with my problems for about a year and a half and I’m coming out on the other side now. This does not have to be for forever. There is hope. You are not alone in this, even if you feel like you are in person.
Wishing you all the best.