First, let me start off with how proud I am of you.
I knew you had some doubts that no one would hire you. I know you felt like a poser. I know you felt like your entire worth was based on finding someone to hire you.
I hope you know now that you were always worth it.
So take a moment to grab a glass of wine and feel proud of your accomplishment.
After that, remember that now it's your time to perform. This client hired you because of the amazing experience you are going to have with them.
Listen to them and hear what they are really saying.
Be responsive. Be friendly. Earn your money.
Continue to market yourself so you have another client in the pipeline.
Document everything you do on this project and go back over it when the project is done. See where you can improve.
Don't forget to be gracious. During and after the project.
Follow-up after the project. Stay in touch. Ask for the referral.
And relax. You've got this. Even if you make a mistake (you can learn from it and make it right).