I have known a lot of flaky people, and have a lot of flaky friends. They promise to do something, and forget all about it. They say that they are going to be somewhere at a certain time and there is no sign of them. In social settings, these people can be a lot of fun - when they show up! When they let you down, they want to apologize, and be forgiven. I can, and do, forgive my friends. Clients are another story.
In a professional setting, a flaky person can wreak havoc. If you are having trouble getting paid by a flaky client, don't give up. What may look like a disaster may just be a function of the disorganization of their life. If you are used to saying, "no problem" to your flaky friends and giving forgiveness, dealing with a flaky client is going to be very difficult for you. But you can do this.
Remember the teacher who insisted that you turn in your homework? Or the boss who insisted that you show up on time? How about the highway patrol officer who insisted that you slow down? Well guess what, you now have to become one of those people! You have to insist on getting paid. If you can't, then just walk away. Doing freelance work as a Graphic Designer isn't for everyone. I started at age 19 and immediately found it much more profitable, and fun, than working for minimum wage. But I had to grow up fast!
Here's what you can do: Learn good business practices, take a sales class at your local community college. Clean up your act. Be clear with new clients about expectations. Learn how to write a quote. Learn how to create an invoice. Learn what a statement is. And be successful!