Since you are now fully aware of your mistakes, take the time BEFORE you present her with the 2nd mock-up to write out a script of what you must say. Don't beat yourself up, treat this as an important business lesson.
Script: Type it out, print it, memorize it. Do not deviate from your "script". Prepare a statement of release from your contract and a return of the deposit (have the paperwork and a check nearby) if the customer does not follow your procedures.
The good news is that there will be no "next time", because you will review your contract, your procedures, and your profit. You have already lost xxx hours by accommodating not one, but two, dummy cake tiers. Although I do not sell cakes, I read a lot and have never heard of any decorator offering anything but a sketch. Many decorators will not release the sketch to a potential customer until they receive a deposit. [They do not want the potential customer to show their sketch to other bakers while shopping for a lower price.] If you are paying yourself $15 an hour and have spent 4-5 hours on the dummy and meetings with the client, you have already lost $60-$75 profit dollars.
Once you approach this with a plan, you will be fine.