It has a little to do with mouse sensitivity, it's your mistake. I for example, regularly block left swing instead of trust. It's my own mistake has little to do with my mouse. Also most noobs (like me) have no clue what to do when they facehug an enemy with 2H sword, we completely lost it and forget what we should do. You need good concentration and idea what to do in certain situation. In most cases offense is best defense but you need to work on attack pattern, some tactics fe. how long to hold a weapon to confuse an enemy, when to realease it and such. Blocking isn't hard thing to do if you're not going against very good feinter.
Mistakes bad players make are:
1) holding attack for too long and not realeasing it on time
2) bad concentration and no attack tactics
3) lack of finer techniques like feinting, chambering
5) bad movement and positioning
Also all above is a lot easier for people up to ping 30 than it's for us with 50+ ping ;)
You just have to practice, don't change classes often, stick to one and you'll get better after some time.
Edit: Try to remember better players. It's easy to recognize them on battlefield, they wear the same armor most of the time. They have one or rarely two attack patterns, it's easy to block if you know what they'll do next ;)