I used to get the lightheadedness and your symptoms too, and after I went to the doctor I found out it was because I had low iron levels, and that can have an impact on your mental health. I have IBS too and I've learnt how to manage it better. For example, there are certain foods which trigger a flare up of my IBS, and for me, its dairy products, excess sugar and (this sounds odd) but white wheat products (like white bread, pasta etc). Also, caffeinated products are known to be bad for both IBS and anxiety.
Anxiety and depression unfortunately can be a symptom of IBS. Some of your mood symptoms will likely be caused by the IBS. I have had the concentration and procrastination problems too. They were again due to my low iron levels, which was caused by the IBS.
I wasn't even anaemic, I just had slightly lower than average iron levels, and that had quite a big impact on me. I went to the doctor when I was depressed, and they did some blood tests and found out about the iron, then prescribed me iron tablets.
If you know you have IBS, you could do some research to find out how to help yourself. There may or may not be certain foods which make you worse. Also, I'm not going to tell you to "avoid stress" like all the websites do, because thats unlikely! It helped my anxiety when I organised everything, so I made timetables, made routines, did homework on time. Simple stuff like that helped me.
I hope you feel a bit better soon.