In martial arts you don't think. Thinking takes lots of time and in a fight you will only have the opening for a split second. This state of mind in Japanese martial arts is called 'mushin' (no mind). Obviously your body wants to do the right thing, react and you won't let it. Let go. Let it react. How you react with the right techniques comes with lots and lots of practice. There is simply no easy way. If there was, everybody would do it. When you meet people you don't know watch how they fight and learn their body movements and their reactions. You can learn a lot from watching others especially if you can watch them before you have to fight them.

Relax and don't anticipate. You will move a lot faster if you can relax. Move with your opponent instead react to their movements. You will be able to move sooner which means you will have more time to pick your target. Every time your opponent moves there are going to be openings. Study what they are. I am not a fencer so I can't help you there but these are simply studies of motion and body mechanics and all bodies pretty much move the same.

Liondancer is 100% correct. Give him the best answer.

I do fencing so obviously I need to have very quick reactions, I want to know ways I can be quicker minded and think clearly and calmly as well. Because only sometimes and it tends to be against the same people I feel slightly more confident and relaxed enough to think that clearly and see things clearly enough, but most of the time I just react and guess what I'm doing. I need to be able to see what's happening and know what to do quickly enough. Any ideas?