One of the main mistakes that the King made was giving Prince Rupert, who was very inexperienced, the role of a commander. He did not know how to make tactial decisions, and this proved to be fatal at the Battle of Marston Moor in 1644. He thought that the Parliamentarians weren't ready, so everyone sat down to eat dinner, then they attacked! No-one was ready so, despite fighting bravely, the Royalists lost and the Parliamentarians gained control of the north of England.
Another weakness was that the King ran out of money. He did have some rich supporters but their funds quickly dried up and most men, in the areas that he controlled, refused to give him anything. As a result of this, at the battle of Naseby (which Parliament won) the King's soldiers were very badly equipped. Also, many soldiers left his army as he couldn't afford to pay them.
Charles I
The Battle Of Naseby, June 1645