[Grammar] I was thinking about doing it for a long time, but then I decided not to

[angelene001]

I've got a problem with this sentence:

I was thinking about doing it for a long time, but then I decided not to.

I don't understand why the past continuous is used here?
I thought that we use it when we want to show that an action was in progress in the past. But we don't use it when we have a finished, completed action.

Why not the past perfect continuous?

I had been thinking about doing it for a long time, but then I decided not to.

 

[Raymott]

There's not much to choose between those tenses here. The're both correct. They convey the same meaning, pragmatically.
"I had been thinking about doing it for a long time, but then I decided not to" has the potential ambiguity of meaning "... but then I decided not to think about doing it", rather than "...but then I decided not to do it". This is because of the function/usage of the past perfect. You could argue that the first sentence has the same ambiguity, but I think it's more pronounced with the past perfect. Using the past perfect when it's not required often has unintended consequences.
Let's make it more concrete:
"I was thinking of going to Japan, but then I decided not to."
"I had been thinking of going to Japan, but then I decided not to."
Here still, the past perfect seems to require some event (in the past, but more recent than the deciding) to terminate it - such as "I had been thinking of going to Japan until my mother became ill." "Until" is better than "but then" in the past perfect.
They are just some random thoughts. Colloquially, the first sentence is as good as the second.