Hmmmm, bipolar? Also, do you take any medication? This sounds like rapid cycling. A mood stabilizer could level things off.

This might be helpful... A 'short form' journal perhaps? When you are having a good week make an entry "I feel good/happy because..." It could cheer you up to read this during the following time when things are down. This is an action recommended to me by my Cognitive Behavior Therapist.

Perhaps I missed it if it was mentioned already above, but are you aware of the trigger(s) that cause both the ups and downs. Are they fairly constant?