I started to read some grammar book , but find difficulty to distinguish between " present perfect contiunous tense " and " past continuous tense " . It seems that both means an action started in the past and will be continued at the the time of speaking .

For example :
I have been reading for two hours .
I was reading this morning .
I was reading since two hours ago .

Which is better ? Is there any difference in meaning between these three sentence .

Thank you for your help in advance .