Mindset of a Successful Investor

Many a times when we invest in a stock the biggest urge is to sell as soon as possible and book profit. In some cases people have this temptation to sell early because of the speculation that the market might fall and the value of their investments will fall and in some cases people have this dilemma that if they don’t ever sell an investment they will not be able to make money and that money is only made by selling the investments?