It’s perfectly correct, and the meaning (in terms of verb time and aspect) is completely different. “I have been working” is about a continuous activity that started in the past and is continuing now. “I was working” is about a continuous activity that started in the past and continued in the past (the speaker thinks of it as ongoing with reference to the past) with no reference to now. Both sentences emphasize the continuous duration, are completely different otherwise. Another option would be the simple past “I worked” — this sums up the whole experience without creating a *continuous* aspect. A fourth option would be the past perfect — this would be about a continuous activity that started in the past and ended at a specific time in the past. Obviously, mastering (or even starting to understand) all the verbal aspects / tenses of English requires lots of dedication, and should be done *in a logical sequence*, at a pace that allows the student to truly assimilate the structures. If you could use professional help, you know where to find me.

Hi Viktoria, sorry i am no teacher so will find it hard to explain specific rules but am happy to talk if you like and do best i can. I am from Australia, so feel free to message if you like. Though will put my best efforts :)

If there's a duration of time expressed (for + X + unit of time or since + point in time), a perfect or perfect continuous tense is needed. All four of the examples you have shown are grammatically incorrect.