My problem is that I go through periods of intense motivation followed by periods of intense malaise.

Ain't that the truth for all of us.

By the way, probably the person most associated with that quote is Kurt Cobain - you know, the guy from Nirvana (who wrote that on his suicide note)? Well, I'm sure you don't need to be told this, but don't do anything too drastic like that in case you decide to burn out!