I agree that you don't seem like a very aggressive person in your dealings with others. But you are being very aggressive to yourself - pushing yourself too hard to study, starving yourself, cutting yourself. So you must have some aggression in you, just not very well expressed.
I know you would not be very good at aggressive sports. But the reason to do them is not because you want to do well at them - it is to get you used to directing aggression outwards. You would want to explain it to the instructor - don't go into too much detail, but just say that you have had eating disorder issues and some self harm, and you need to do this to get out of that - to help you be more
aggressive, or at least express it better. Actually you would get the most value from it precisely because
you are not likely to be very good at it.