Obsessed or Obsessive?
Cambridge may have oversimplified the matter somewhat. The difference is in the suffixes, which both operated upon the root word obsess.
obsess To excessively preoccupy the thoughts or feelings of; to haunt the mind persistently. 1
-ive An adjective suffix signifying relating or belonging to of the nature of tending to; as, affirmative, active, conclusive, corrective, diminutive .1
-ed The termination of the past participle of regular, or weak, verbs; also, of analogous participial adjectives from nouns; as, pigmented; talented.1
Thus the word obsessive relates more narrowly to the tendency to frequently think a certain thought or feel a certain way.
The -ed suffix is a little more complicated as it acts as a past participle.
Participle (Gram) A part of speech partaking of the nature of both verb and adjective; a form of a verb, or verbal adjective, modifying a noun, but taking the adjuncts of the verb from which it is derived. In the sentences: a letter is written; being asleep he did not hear; exhausted by toil he will sleep soundly,written being, and exhausted are participles.1
Since participles have the partial nature of a verb, it indicates some sort of action which implies an agent to cause the action. This is not so different so far but participles often denote an ongoing state of action, and past participles in particular further note that this state has at the least already began, if it has not yet also subsisted too.
Hence obsessed indicates that there is obsession occurring within a certain period of time. This places emphasis on an act of obsession itself and its actual cause, rather than having the latent ability to become obsessed or potential to cause obsession like the word obsessive does.
1Webster's Revised Unabridged Dictionary 1913
I will try to give you a more intuitive, less theoretical grasp of the difference. Think of possessive and possessed. I think they are easier to distinguish, but they could give you a useful model to hang your hat on.
The character in the horror movie was possessed by an alien creature.
Small children can be quite possessive about their personal belongings. It can be helpful to prepare for a visit from a young friend by inviting your child to put certain key personal items in a closet before the friend arrives.
When you're obsessed about something, it is almost as though you were taken over by an alien creature.
When you are obsessive about something, you have a certain tendency. That is also a bit like the child who's possessive about his things. ("Mine, mine, mine!")
There seems to be some over-complication attributed to this.
"Obsessive" has its roots as a psychiatric word, involving both anxiety and mental disorders. Its use in common speech stems from this beginning, but it still maintains a medical/clinical connotation.
Most obsessive tendencies can be traced back to childhood events or traumas.
Do you think ironing my jeans makes me look obsessive?
His love of movies tends toward the obsessive. (He goes to the cinema 365 days a year.)
You're obsessive about the lives of celebrities. (I don't want to say stalker...)
"Obsessed" is just the past participle adjective construction of "obsess". As such it implies preoccupation and fixation.
I am obsessed with cat photos on the Internet.
Obsessed with the new trilogy, he barely looked up from his books all week.
He seems obsessed with movies. (And chooses to quote them far too often.)
You're obsessed about the lives of celebrities. (Weird, but not clinically weird.)
You are, I hope, only obsessed with the recognition of English phonemes. Unless you get the grammar shakes...just one more etymology, man...I'm good for it...