I'd suggest that as you have such difficulty in making a fist, instead just develop your palm strikes, they have a number of advantages, you'll be less likely to injure yourself when hitting someone, your blows don't need to be as accurate to still connect and either damage or distract your opponent and also, one of the more useful blows that can be used in a 'streetfight' I've found, is basically jabbing at the centre of the opposite players face with the heel of the hand, with a fist, the movement is slightly slowed, and, unless very accurate, it's less effective.
Just for a bit of fun, spend a little time practicing hitting the heavy bag like this, keep your habds low and relaxed, then just hit at the bag at around face height, landing with the heel of the hand, after a little practice, you'll see how easy it is to do this fairly qickly and accurately.
Also remember that in most 'street' type situations, your assailant, if he's expecting you to hit him at all, will most likely expect you to telegraph some kind of haymaker, as, that's what most people do in such situations, so the straught palm straight through the middle is a good opening shot, it's both unexpected and difficult to block.
Have fun.