If you are fighting as a southpaw, you are killing your ability to land effective punches by fighting in straight lines. You really need to learn how to circle, especially to your right. Back when I first started boxing the saying was that a jabbing and stepping southpaw is real hard to beat. By doing that you are making your opponent back/turn in such a way that he opens himself up to your straight left, and you are in a position to land it. Unless, of course, he is smart enough to get low and turn with you like James Toney did to Michael Nunn, but that is a different subject...
The punch that makes a southpaw really effective is the right hook. You can create angles and shoot that left hand, but that right hook comes from no where, and you can set it up off his straight right, which is the 'text-book' punch against a leftie. Of course, if he's smart he's going to be looking to time your left hand with his left hook in the same manner. If you watch Pac, he shoots that left hand, but when he is looking to hurt you, it is the right hook that does it. This is true of other southpaws going back to Tracy Spann, Andy Ganigan, Chiquita Gonzalez, Hagler and Moorer.