I don't know if this topic matches with subreddit, but I have no idea where else to post . Just a little bit of intense mental activity make me feel out of energy and get jittery and unable to concentrate. I have always had this problem. I also have always had huge anxiety issues, probably GAD, which I quite managed to handle with pills and experience... This problem probably triggered my binge eating disorder, which in the end managed to deal with too, but it returns if I am not careful. 
I don't have this issue with physical activities tho..