The key to a good knife self defense is controlling the attacker’s knife hand. It should be your main focus but not the only one.
Always anticipate his next move and try to stay ahead. Keeping a good distance is also a good idea. Never attempt to close the gap unless you know what you are doing.
Parry his attack and hit him anywhere as hard as you can. To maximize your blow, hit his vital parts such as the throat and face. if that is to high for you go for the groin.
Be aware of his other hand though. You will never know what else he has up his sleeve.