We use "think about" to speaking of something occupying one's thoughts:
a: I've been thinking about her all day.
We can also use "think of" to mean the same thing:
b: I've been thinking of her all day.
Thus, in your example, both "about" and "of" work:
c: I often think (of/about) the time we spent in Rome.
(I would leave off the phrase "that I can't forget". If it were forgettable, you wouldn't often think about it.)
However, we also use "think of" (but not "think about") to speak of having an idea come to us suddenly:
d: I just thought of a simple solution to the problem we've been agonizing over. Please set up a meeting this afternoon so I can explain it to the staff.
So, "think about" means one thing, and "think of" can mean the same thing, but it can also mean other things. I hope this helps.