I'm pretty much always like this. The only reason I do end up revising for exams is due to the stress of being unprepared and failing miserably. I can barely focus in class - I haven't learnt a lot in school for this reason and end up learning most of the content myself before exams.
I've found that constructing links between anything you're doing currently e.g. revising for exams to something nice you experience, regardless of how ridiculous those links are, helps a bit. An example would be seeing a picture of a nice place and thinking 'I'll go visit that place once I'm financially secure after I have a great job which I've gotten by studying hard for my A-levels'.
The idea is to come up with some nice hypothetical scenarios and linking them to whatever you're doing.