I find that if I try to motivate myself through 'love of success' I will either become complacent in any successes I do attain, or de-motivated through lack of confidence (believing that I cannot achieve my desired ends).
Through fear (I'll warrant, a negative mentality) I am able to convince myself, that poor marks will result in detriment to my own successes in this stage of life, and disappointment, both personally, and from those around me.
When especially worried, the adrenaline gets pumping, and I can study like a madman (that is, relative to my own shabby standards
) - this helps me out a great deal.
On the downside, however, this fear does sometimes impact on my overall happiness - when this hits, I try to remind myself of the light at the end of the tunnel - that when early November rolls around, I can goof off and fuck around as much as I like, and that this fear, is, all in all, a small price to pay for prosperity in the longer term.