The customer is always right, but often they are a complete dickhead as well. You have to remember that you have let them down badly, and they are the injured party here, not you. How much did she want as a cancellation fee? If it's a reasonable amount, you really ought to have offered it as a gesture of goodwill - small businesses and self-employed people especially need the goodwill of local customers. At the very least you should have offered to clean the place for free next time.
What do your terms and conditions say? (You do have them, right?)
Texts do get delayed or go missing (to the people who say "I've never had one go missing...", you wouldn't necessarily know... because you didn't receive it...).
You live and learn, next time phone them. Your comment that you find it hard to be assertive is very telling - sending a text was easier for you than cancelling by phone, because you'd have to speak to someone to disappoint them.
Avoiding doing something difficult only reinforces the feeling you can't do it and, as has happened here, it's made your life more difficult. You might find something like this useful. Learning to be assertive is like learning anything else. You start by being shit at it, and gradually get better. Some people are naturally good at it, but then they probably struggle in other areas. If you're not naturally assertive, it's only by exposure to "scary" situations that will help you overcome your fear. You'll have good experiences and bad experiences along the way, but don't berate yourself if you have a weak moment or an outright disaster when trying to be assertive. If it all goes wrong and you feel hideously embarrassed, the sun will still rise the next morning, you'll still be here, and you will be able to look at yourself in the mirror and tell yourself that you are actively trying to improve your life, which is more than a lot of people can say.