The best thing I do is to not push too far. I set myself small goals and then have some kind of small reward system, and allow myself a certain time to panic/calm.

Eg. I decide to finish reading a chapter then I will spend 10 minutes off. I will have 5 minutes to think about what worries me and write it all out to get out of my head, get a drink or something to eat and then dedicate myself to another 20-30 minutes of study.

It sounds weird but I find that if I just keep trying to study and don't acknowledge what I am feeling/thinking I end up stressing so much that I don't get anything done.