Why can't I focus or remember as well as I used to?!

I am 21 years old and it doesn't feel like I can concentrate on *thinking* about a certain task (i.e. reading, mentally solving a problem or puzzle) and it feels like I have no long term memory (such as knowing material back and forth during a class but remembering nothing about it two weeks after). Possible depression or anxiety could certainly be a factor, among a host of other potential symptoms like nearly debilitating fatigue (and maybe even hypochondriasis?). I used to be quite brilliant and would soak up knowledge like a sponge, but in just a few short years it just feels like my mind has changed. It's really affecting my schoolwork and I fear will destroy my future prospects. Anyway, where might I start trying to fix this? Name anything: supplements, tests, music therapy, tarot card readings, you name it.

I do need to mention that I'm pretty sure exercise will take care of almost all of my problems, but I've never been able to keep up a regiment because of my fatigue and probable depression.