Welcome to 3FC!
You can do this. I know that, because I've been reading through this site, and have seen the success stories of people that when they began sounded just like you.
We've all been there, the amount of work we need to do seems so epic so large, that even thinking about it just makes us more frustrated and slip even further from our goal.
Here is the thing. Sometimes we need to not focus on the larger picture and just work within a few small goals. Our success with these small obstacles (like for you Pop) will help give us the motivation to make more goals and have more success.
My suggestion to you is to do 3 things.
1) Focus on April 1st. There are challenges that end April 1st in some of the other forums, joining one and posting often will help with your motivation. If you can incorporate some healthy changes into your life until April 1st, then you've already beat one of your barriers, of not thinking you can stick this out for more than a month.
2) GIVE UP THE POP
Not cold turkey, but switch out the coke for coke zero. It won't taste the same, when I first started drinking it, I thought it was a vile poison created by Coca Cola to drive me back to coke. But since I refused to drink away my calories it was Coke Zero or water.
Speaking of water, once a day when you really feel like a pop, have a glass of water first. You might be able to kill the craving right there. Every week, add another glass of water instead of a pop. By April 1st, you could be down to a few diet pops a week.
3) Start tracking everything you eat. (if you don't already). If you have a smart phone try My Fitness Pal or Lose It. There are also websites that can help you track your food intake. This will help you figure out what you're eating and where you can make small modifications that will give you results.
4) I know I said 3 but I thought of one more
Don't be so hard on yourself. We all make mistakes. We all fall off the wagon, and we are so inundated with weight loss information (low carb, paleo, atkins weight watchers etc) that is can seem very overwhelming. Some small changes will get the scale moving in the right direction, which will help keep you motivated.
Plus we're all here for you too. So good luck, and don't forget to join us in the other forums!