Step 1: Food Diary

The first step in trying to change your eating habits is to start a food diary. You need data.  It can be in a notebook, or on the computer, or on your phone, it just has to include 1. what you eat, 2. your digestive response, and 3. how you feel.  It can include more, like exercise, sleep, menstruation, or medication intake, but it has to at least include those first three elements in order to give you information about how food is affecting you physically and mentally. A rough food diary template looks like this:

You can format it any way you like, this is just an example.  For myself, I just use a composition notebook that I keep in my kitchen. Write down everything you eat and how it affects you for at least a month before you try to make any behavioral change.
It is very important that you start understanding the connection between what you eat and how you feel, because every body is different: every person has different digestive evolution, different metabolic type, and different deficiencies in hormones, neurotransmitters, and enzymes.  Only you can discover what affects you negatively or positively, and you cannot rely on guilt and willpower, you must rely on the data. 
Guilt is counter-intuitive to the process of change. It does not make you want to change more, it makes you feel more weak and worthless.  If you eat a donut and all the while hate yourself because "Heather just told me this is so bad for me" you will only make yourself unhappy and uncomfortable. The dopamine surge is powerful when faced with food!  And a person feeling unhappy will be more susceptible to letting the dopamine win.  So instead, rely on the data.  Instead of white-knuckling abstinence with a giving-in to have just one bite, and then since you gave up might as well eat the whole thing, we want your experience to look like this:
"According to my food diary, when I eat white flour alone I get bloated, and when I eat sugar alone I get moody, and let's see when I eat flour and sugar together I feel happy for thirty minutes then I have a stomach ache and I get super irritated and then tired.  Let me check the ingredients on this pastry, yes, there is 26 grams of sugar and a total of 30 carbs, and refined wheat flour is the first ingredient, and oh! it's cooked in soy bean oil which on my food diary shows causes me to feel nauseous.  This will make me feel sick and cranky for sure, I choose to not have the pastry."
This will help you to have tangible proof that what you eat affects you, which will help you change with truth and self worth rather than guilt and shame. This also gives you lots of information when people ask "Why aren't you having dessert? You don't need to lose weight."   Furthermore, perhaps certain food that others deem unhealthy does NOT affect you very negatively, how will you know unless you have the data? If soy bean oil (vegetable oil) causes a flare up in pain for one person it does not mean you will have the same reaction.  As has been stated many times in this narrative, some people are far more sensitive to sugar and refined flour and "inflammatory foods" than others.  You can make the change, start with the food diary.

"In God we trust, all others must bring data." --BYU Autism Department