Confidence
Anyone who has any experience of teaching juggling to others will probably have noticed the lack of confidence with which many people approach the learning of this new skill. Some people are even convinced from the start that they will never be able to do it, and often this attitude is the only thing preventing them from learning with ease.
One of the most accomplished and well known jugglers today is an American called Anthony Gatto. He started juggling at the age of 5 and by the time he was 11 he was among the best in the world. In his early 20s he had a string of Guiness records to his credit, and frequently headed the bill in Las Vegas variety shows. I once heard a very experienced juggling teacher say that he believed the reason Anthony achieved so much at such an early age was that he was too young to know that the things he was trying were 'impossible', and consequently he learnt to do them.
I firmly believe that confidence is more important than natural ability in the making of a good juggler. At least once every time you practice, spend 5 or 10 minutes on a trick that you consider to be far too difficult for you, e.g. if you are trying to learn 5 balls then have a go with 7 for a bit. Doing this has two advantages - firstly, having tried the difficult trick, the one you are seriously working on will seem easier in contrast, and secondly when you actually get round to practising the difficult trick seriously, it won't be so intimidating.