difference between I was thinking and I have been thinking

Why do some people say "I was thinking and some, on the other hand, say I have been thinking. Since an English is not my native laungage, I am still little confused as to which of those two is correct and when each one of them is used.

Any help in clarifying this confusion is greatly appreciated.

"Have been thinking" suggests something that you've been thinking about recently, that you're still considering, that has a relevance to what's happening now or to what you're about to do or say, etc.

1. I've been thinking about selling my car. Do you think I should?

(I'm considering it ... my thoughts might result in my selling the car.)

Contrast with:

2. I was thinking about selling my car, but in the end I didn't.

(I'm not thinking about this any more ... it's not affecting the present situation.)

In practice, though, "was" is also used in sentences of the first type:

3. I was thinking about selling my car. Do you think I should?

There isn't a big difference between (1) and (3), but, on balance,  (1) more strongly suggests that you've been thinking about it recently, and the thought is still fresh in your mind. (3) might suggest, say, that you thought about it last month.

Although, as illustrated, things that are still relevant may use either "was" or "have been", things that are now settled require "was". So, "I've been thinking about selling my car." would not be appropriate in (2).

Sometimes in a discussion, you really disagree with what others are saying.  You want to make your point without being offensive, so you use "was thinking" to suggest that perhaps you no longer disagree. You are thus able to state your case without being argumentative.

You can carry this one step further, and use simple past.  "I thought you said you'd have the money for me today."  You really mean "I think (know) you said you'd have the money for me today."

  - A.