If you don’t honestly believe that you can add value to your clients’ businesses, then you’re going to feel dishonest in your marketing and sales efforts.
If you feel dishonest in your marketing in sales efforts, then your prospective clients are going to sense it.
If your perspective clients sense that you are being dishonest, then they are not going to buy from you.
The best way I know of to break this vicious cycle is to get results oriented testimonials from past clients.
When you are confident in your ability to deliver results, it makes your life a lot easier.