i would consider myself somewhat of a heavey hitter, but i do hit the heavy bag just about everyday, i also put weights on my wrists and ankles after i warm up just to help me with speed and power. you might want to try that out. you should be able to get strap on weights at any sporting good store.
i have also found that you dont nessecerily need a super powerful punch. a well placed punch to the throat, groin, jaw and a few other key pressure points will do the trick. try to find a source of target areas of the human body, then get a Body opponent bag or what have you and focus on those select areas when you train.
also there are many soft tissue areas or sensitive spots like the eyes, ears, nose, ribs, groin and knee joints that are pretty easy to get to. in a fight. dont be afraid to learn some anatomy. it will come in handy someday.
"I wanted you to see what real courage is, instead of getting the idea that courage is a man with a gun in his hand. It's when you know you're licked before you begin but you begin anyway and you see it through no matter what."-HARPER LEE