When you are a very ambitious person, you tend to want to do everything. But learning to say No can be a good thing, especially for your productivity. The consequences of taking on too much work is you run the risk of producing half-baked or poor quality work. This impacts on your reputation, self-esteem and your bottom-line (because clients may not come back). Saying no does not mean you will lose clients, if anything, they will have more confidence in you. Saying yes then going back with excuses will have a negative effect.
Know your capacity and understand the importance of saying no in order to maintain your high standard of work and reputation. It is better to turn down work than to produce poorly done work.
Checkout my post on effective time management.