I was having a conversation with a friend yesterday who has decided to use a diet tracker app to watch what she's eating - she used it for the first time after what she thought was a "healthy" day, and was shocked to find that she'd consumed an exorbitant amount of calories and a pretty limited range of nutrients. She's not someone who freaks out or goes into a guilt spiral if she's eaten "too much" - she was just shocked at how unconscious her eating had become, that she didn't really know what or how much she was putting into her body. 
I think a lot of us have that issue - it's so easy to miss out on vital nutrients or buy into marketing (eg iced tea is often marketed as a healthy alternative to soda, but it's absolutely full of sugar).
The key is finding out what you're lacking in, reading labels, learning what your specific body needs to stay nourished and healthy and just being mindful of what you put in it. I've got a few different tests I use to find out where we're starting from, so I can make recommendations. 
And once you have that knowledge and a plan - simple things like writing amounts on a bottle so you can ensure you're getting say, enough serves of folate, can make a world of difference to your health. Take one small step every day, and you'll be amazed where you end up.