Hello and thanks for the welcome and question.
I think definitely my proneness to depression. When I'm going through a period of depression it significantly lowers my productivity, that then makes studying/work very difficult. I have been on and off anti-depressants for the last 10 years, since my brother's death (2012) I have stayed on them consistently, they help a lot for me personally.
I was trying to remember when I first experienced what I now know as depression, I think the earliest time I noticed it was at around the age of 6 or 7, this was when I realised that; my parents, brothers, grandparents, extended family, friends and also myself, was going to die someday, and I couldn't do anything to prevent it. It still bothers me but I try not to think about it. Breakdowns in routine is another general trigger, the worst is through illness such as getting a bad cold or the flu. Once I'm out of my routine it takes me days or weeks to fully get back to my maximum efficiency.