What's the best form of fighting/self defense I can learn?

If you want to learn self defense quickly I wouldn't go with Karate or tae kwon do unless the teacher is focused soley on self defense. They take years to learn and are more sport oriented in the U.S. Krav maga is good. So is muay thai and kick boxing. If anything they will give you more confidence which makes you less likely to be attacked. Your small but Krav teaches dirty tricks (in a good way) that could help. You could carry mace or a stun gun if all you are worried about is defending yourself, they can be really effective. At your size you might have a hard time grappling with or knocking out a 200 lb man no matter how good your technique is. Keep that in consideration and chose something that puts the maximum amount force on the weakest areas of the body.