Sounds like classic Bipolar Disorder symptoms. People w/ bipolar or even people who are just depressed often have trouble with attention span/racing thoughts. For this reason these illnesses can be confused with ADHD.

Bizarre thoughts... switching topics... moodiness... impulsiveness (shopping sprees)... What you described in your post are all symptoms of mania. Being on a mood stabilizer (like Seroquel, Lithium or Depakote) should drastically help with these episodes. Antidepressants, on the other hand, are a bad idea. They'll only trigger more mania & you could end up hospitalized. It's happened to 2 of my friends.

Dig up some info on bipolar & you'll see for yourself that the symptoms you're having are typical of a person with this illness. Then go about obtaining proper treatment (no SSRIs!!!).