One of the difficult things (of many) to understand about IBS is how, if it is all in our heads, (as some medics think and don't hold back about telling us the same) what can we do about it? My first comment is any health professional says it's all in our head, change GP or Consultant, they are behind the times - big style.
Some years ago I visited a guy called Micheal Mahoney up in Cheshire. He answered this question for me, so completely that one conversation changed my life! He helped me with my anxiety too, but thats another story.
Anyway he is the guy behind the IBS audio Program 100, I got a copy of it, and it did good for me.
I see he has the introduction of this program on his site, I would suggest if you want to know in detail, why IBS makes you feel low in emotion as well as rough physically, you take a look. Good down to earth advice and none technical, easy explanation all through. I have no vested interest, only to help on this board where and how I can.