This is a common problem when learning Morse that if you miss a character you tend to think about it and miss the next character and suddenly you have missed a sentence. Get into the habit of putting a dot where the missed character was and move on and don't think about it. Thinking about characters is bad news when trying to take Morse at a decent operating speeds and as you progress you will tend to miss less characters as your proficiency increases till you are getting 100% copy.
A reflex action is, that when something happens you automatically react to it like braking when driving a car, thinking about it could be expensive as you are taking longer by having to think about it. Reflex action or instant recognition in Morse is when you hear a character you automatically record that character without thinking about it and Nancy WZ8C sent me a very good article about it which you can read hear. You need the reflex action in Morse to attain higher speed so stop thinking.
This is a common question and I presume that you are learning using the Slow Morse method and possibly Crib Sheets. You can struggle for months or years to increase your speeds, but it is better that you start again from scratch using the Koch Method and although can be a bit of a bore you will soon find that your Morse profiency increases to a usable speed on the bands.
If you go back to the compulsory Morse test, it was 12wpm but the average speed of Morse operators is around 18wpm. True to the amateur spirit faster operators will slow down to the speed of the calling station but beware of your sending as some operators send faster than they can receive. If this happens with your own sending then rather than keeping asking the other station to repeat, just ask him to send a little slower "PSE QRS" and I am sure they will oblige.
Although Crib Sheets can be good for learning Slow Morse, they are only useful to a speed of 10wpm then you will have to unlearn using them to get further so technically they are a waste of time. Slow Morse is also a waste of time and effort as you will struggle to get past 10wpm. If you want to learn Morse quickly then use the Koch Method
It has been recognised that the best and quickest way to learn Morse Code is by using the Koch or Farsworth Method of learning and I tend to go more with the Koch Method where you start with two characters and gradually increase the amount of characters. You will find information on the Koch Method and suitable Software on the "Guidance" section of this website.