Yea, I mentioned earlier that the SAO is not a gun for novices, and the 1911 falls into that category. What you experienced today is the drawback of that style, it's trigger is very light and that can lead to a "negligent discharge." I would stay away from ANYTHING in .4 caliber and .357 magnum; they will kick worse than the .45.
22 guns are great for beginners to learn on, but are not powerful enough to make a good self defense round, that is why Caynracer is so pro-9mm; it is powerful enough for self defense but not so powerful that it is too much for a novice to handle. I agree with him on that point, .38 special is in a similar category.
As to why bullets that look the same are more and less powerful it has to do with how much powder is in them. More powerful rounds have more powder and less powerful ones have less powder. Guns are built to handle the rounds that they are designed for and no others (in most cases), doing otherwise can result in a blown up gun.
I have refrained from recommending a handgun up until this point because, as the others have said, the best idea is to get training and try a lot of pistols. Based on everything that you have said though, I do suggest that you check into the Smith&Wesson M&P in 9mm. It comes with changeable grips so you can make it fit your hand and should be fairly easy for you to handle.