No, you don't suck, you just have to train and understand game mechanics, it's all about timing, footwork and type of attack.
timing is important, that means try to anticipate, also you must know that sometimes after the first attack you are stun and you can't attack before the second attack.
Foot work is needed, try to circle around your opponent and you will find some weak points.if your opponent have a better reach than you then stay close to him, but after an attack you can backpedal and release and attack, when your opponent strike back you will be unreachable.
Left slash is the key, fast and direct to the head. If you try a right slash against 2handers 80percent of times 2h will be faster due to animation mechanics. When you are behind your adversary(footwork) you can try overhead slash.