"It's just a little bit of something bad for me! So little, it doesn't even count!"
If only. If only life consisted of what we can get away with! In reality, it is what we do consistently, intentionally, and with self-awareness that produces the true content of our lives. Everything else is just a side effect, a byproduct...accidental.
Fitness and nutrition are no exception. They don't happen accidentally, or without intention. When we achieve them, we accept credit. When we falter, we accept responsibility and move on.
Part of the purpose of working with a personal trainer is to establish accountability. Believe it or not, your decisions mean something! Each decision---whether to choose donuts over oatmeal for breakfast, or to drink alcohol before driving home---affects your life in subtle or not-so-subtle ways. And every decision counts. On one level, this feels like an unfortunate fact, because it makes life harder.
In 9/10 of cases, what you eat and how much exercise you get comes down entirely to your own decisions. Sure, life is a complex and busy ordeal. But it up to you to decide how much time and calories in a day will be spent productively, or destructively. And if you fail, at least you can understand why and try to address the problem.
If you can't acknowledge this, though...if you're freaked out by the idea that your own health is your responsibility, and no one else's, then there's no way to address the problem and improve, even incrementally.
In other words, if you can't be honest with your trainer, there's a good chance you're not being honest with yourself.
But there is a bright side. Remember what I said above about decisions?
"Every decision counts. On one level, this feels like an unfortunate fact, because it makes life harder."
This is true. On another level, though, this is what makes life rewarding and fulfilling: exerting control over our lives to the extent that we can with our own conscious decisions.
Tell your trainer the truth. Allow him or her to help you towards a goal, towards a greater version of yourself, a version that recognizes the value of your own decisions and puts them into action, for your own health and for a greater sense of freedom.