Stop making excuses for yourself! YOU are the expert, train yourself to come across as a self-confident professional, and your new clients will start taking your word as gospel (as long as you actually deliver what you promise).
Remember, you are not competing with high school kids that need pocket money. 'clients' - I use the term loosely here, who are looking for cheaper than dirt are not worth pursuing. Brush up your presentation, you seem articulate enough, wow your prospects with your expertise in many fields: from database design to dynamic website programming and standards-compliant design that follows industry best practices. Offer business analisys, banking on 10 years of expertise designing succesful online sales tools. Give advice on social media, upsell yourself. - php, mysql, css doesn't mean a thing to the average shop owner or small business. They want to hear how you can help them get more business.
In order to make them trust you enough, you'll have to learn to speak to them in words that resonate and maybe juice it up a little. Your sales skills suck, and you'll have to learn that part too. Unfortunately, that's part of being a one-man shop.
Maybe there's a community college near you that teaches presentation skills, public speaking, self-marketing? Whatever works for you, but first you have to believe that you deserve the better clients...