If you don't believe your product or solution is the right choice for your dream client, you don't stand a chance of persuading him that it is. If you don't believe that you provide more value than your competitors, that you're the best choice your dream client can make, then why should he believe it?
To excel at sales, you have to be sold on two things: First, you need to be totally sold on what you sell. You must believe wholeheartedly in your product or solution. Second, you need to be totally sold on yourself. If you walk around carrying fear and doubt about your ability to deliver, then your dream client will pick up on that and feel the same.
There are some sociopaths out there, who have the ability to persuade others even when they are not persuaded themselves. They are slicks, con men, snake oil salesmen. You don't have to be any of those things to be completely sold on what you sell and on your ability to deliver for your dream client.