A southpaw with a weak left hand is a victim waiting to happen. I'm suprised that people are countering your weak left with right hands.The counter to a southpaw straight left is a left hook; don't see it coming and you turn right into it.
I would suggest finding some source that credibly details the mechanics of throwing a straight right hand and mastering it, with your left of course. Punching is technique.
If you can learn how to turn guys into it, a southpaw right hook is a substantial weapon. Keep in mind that most go right hand happy when fighting lefties (Why???); just as the counter for a straight left is the left hook, counter their right with your right hook.