If you truly enjoy the subject then procrastination should not be a problem so maybe that is something you must take into account.
Stress plays a big part in getting stuff done so you probably need to get stressed more. Obviously don’t get yourself too stressed otherwise you wont end up doing anything. All the straight A students I knew used to get themselves stressed all the time over pretty much everything. The other types are those that are simply naturally gifted which is a rarity and those that work on efficiency avoiding procrastination at all times. This type of person normally does not tend to do things like watching TV, browsing random websites on the Internet very often etc. Instead they learn something new or study, basically anything they consider productive.
Stress is probably the easiest method for you, talk to your fellow students about how far there into their work. Normally when they say they are 50% through it and you haven’t even started it should get you a bit stressed.