Hi, a short answer:
It might be Mast Cell Activation Syndrome (MCAS) or else mastocystosis, especially because of "terrible IBS, sweating, breathlessness from mild activity... terrible heartburn after every meal (most days with reflux as well)".
The digestive tract, skin, and lungs are lined with immune cells called mast cells. Mast cells secrete over 200 biochemicals that control other cells. It's very complicated and not easy to diagnose. Many sufferers see several docs over several years in frustration.
"headaches, inability to concentrate, difficulty focusing my eyes"
Those as well as the fatigue, can be downstream effects of immune chemicals such as histamine and prostaglandins. The "inability to concentrate" is commonly referred to as "brain fog".
One note on "terrible heartburn": the common antacids such as Pepcid are really histamine blockers.
Intertwined with all of this might be exposure to molds or other toxins which trigger the mast cells. Do you live where there is mold?
Who wouldn't be depressed after suffering all that you have for years, right?
A doc who is unacquainted with this might scoff, so then you can show this article on the topic in the ultra prestigious New England Journal of Medicine. It's for-pay but every doc would likely have a subscription.
"Mast Cells, Mastocytosis, and Related Disorders"
Being published in there shows it's not just some flaky thing. Let me know what the doc says. Make sure your husband takes a pad & pen to write notes. Good luck.
"I live in a reasonably rural area though."
Sensitivity to pollen or whatever could explain your childhood symptoms, then eventually things got much worse for you.
Have you tried and antidepressant or an anti-anxiety medication? From your symptoms listed I would guess anxiety and depression are involved with all of this pretty significantly
Okay, sorry if you say and I miss it. Are you female?