Welcome on board the Bersa Pistol Forum from the Lone Star State.
I really not sure what kind of problem you are having with the trigger spring but if you mean the spring in the picture it could be that the drawbar is pushing on the spring knocking the spring off. If you look above the Drawbar you will see a round circle which is a roll pin, if the pin works loose it can push on the Drawbar causing the spring to come off. Also check out this video which show how to assemble some internal parts. Bersa Thunder 380 Lower Receiver Reassembly part I.flv - YouTube
I don't know if this helps you but there are other people here with more expertise than me. Let me know how it turns out and good luck.