My story is very similar to yours. Without going into as many details as you have here, the main point is that I am afraid of becoming constipated, and so I (consciously and unconsciously) place a great deal of importance on having a good bowel movement everyday. Perhaps it's because I place so much importance on it, that I have built up anxiety about doing it. Diet and exercise are quite helpful, but it's only half of the puzzle. However, I think it's worth paying attention to this half, because it's a half which can be controlled.