I suggest you discuss this with your doctor. Long term use of SSRI drugs like Celexa can mess up your dopamine system.

Are you having any muscle cramps, neck or jaw spasms, restless legs? These side effects are called extrapyramidal side effects and involve abberations in dopamine.

Low dopamine in the brain can cause poor learning response, lack of motivation like you describe, since dopamine is a reinforcement neurotransmitter.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2674976/

Sometimes doctors add in Wellbutrin (which is not an SSRI but a dopamine acting drug) to a SSRI regimen. However, it may be decided by your doctor to wean you off the Celexa...if so I think it would have to be a very slow taper because of the length of time you have been using it.

In any event, you need to discuss this with your doctor.