The enthusiasm will always slowly move back to the default point, which is 5. The further away from this point, the faster it will move back.
That's the reason your enthusiasm drops quite quickly from 13 to 9, but very slowly from 6 to 5.
Playing a normal season CT (which is no option in the playoffs) decreases your enthusiasm with 50%.
Playing a normal season TIE (which is qua enthusiasm equal to playoff normal) increases your enthusiasm with 33%.
Playing a normal season normal (which is qua enthusiasm equal to playoff CT) will have no direct effect on the enthusiasm.
During the daily updates your enthusiasm will slowly move to 5, thus it will get better when you've got 3, and it will get worse when you've got 8.
My guess is that when you play one week of normal/playoff CT, and you will start with 9, the enthusiasm level will be around 6-7.