I was actually reading something that other day that was relevant to this. What it said made good sense and might give you hope.
Our intestines are home to a great many bacteria - of many different types and variety (is that redundant?). When you eat a certain diet (ie junk food), those bacteria proliferate and send signals to the brain (yes the bacteria does this!) that trigger reward (ie pleasure) response. So eating these foods that THEY want makes you feel good in the head (while making you feel like crap in the rest of your body). When you try to change your diet to something healthier, those same bacteria that thrive on sugar and junk food begin to starve and die and will send signals to the brain trying to drive you to feed them the food they want. You need them to die off and/or go dormant. You want the bacteria that thrive on healthy foods to grow and take over your gut. That takes time - time for them to establish a connection with your brain and time for them to dominate and drown out those old bacteria.
I recently went through this. I switched my diet to mostly vegan, cutting out all junk food and meat (except for fish sometimes). At first, eating salad every day, a couple times a day was tough. it took a lot of mental willpower. After a few weeks (yes weeks) it became much easier and I actually started enjoying the taste quite a bit! I'm nearly 40 and have NEVER liked salad. Suddenly I'm enjoying it and when I have tried non-healthy foods, I don't receive the same level of "reward" that I used to. I've always been somewhat healthy though so it wasn't a huge transition for me, but it was still tough.
My suggestion? Start with ONE THING to change. Put down the burger and start eating a salad. give yourself something to hold onto while your body and gut adjusts the change (ie don't quit the soda yet). Once you've adjusted to that one thing, and are enjoying it, you'll find it easier to change the next thing, and then the next.
Remember it takes at least 21 days to create a new habit - but like 90 days to reprogram an old habit.
Good luck!