but I often wonder if some of it is in my head.

I think a lot of it is, or at least, the head makes it much worse. I had issues for so long that I was able to run experiments for many different things. One thing that does help though that isn't head related, is cutting down carbs significantly for a couple months, then slowly introducing again, but keeping them pretty low still.

I also think there are a lot of things I have missed out on because I didn't want to be in a position where my stomach problems would be an issue.

This I understand a lot. I feel I missed my university years and much of my mid twenties because of it. It has only been in the last ten years I have been freer to move again, though it is still always on my mind, somewhere at the back there.