I feel for you, I have been in the industry for 20 years and have only recently come accross clients who want glamerous nails but who chew the skin, nibble the free edge, pick at the emhancement and come back and blame you. It can be soul destroying but what I do now is:- while I am doing the nails, I make sure I chat about what not to do with your nails in a nice and sometime humerous way. One of my stock phrases is "rember, your nails are jewels not tools so dont go scraping stubborn things off with them or use them to open cans of fizzy drinks" another thing I say is that you must always treat your nails with care as strong detergents can cause the acrylic to degrade, then I say " so remember not to put your hands and nails into anything you wouldn't wash your hair in" then lastly as the client is about to leave, I often say "now just one last thing to remember, please make sure that you keep your eyes on the road driving home, I wouldn't want you to crash your car while admiring your nails" Can you print out an aftercare slip with how to look after your nail enhancements and give it to the client to make sure that they know that the can't just have their nails done and carry on biting, picking or dipping them washing up liquid or bleach etc. It will make you more confident if you cover all the bases, especially if you suspect that the client may be trouble. A classic sign is if they have tried multiple paces and always found fault. And never be afriad to say no if the job looks like it may be difficult. You are the professional with the knowledge and the client must be guided by you.
Hope this helps
Gilly