I think you at least know the mindset you should switch out of. Try to start thinking of everything in terms of denying yourself the pleasures of your bad habits right now. If you lapse, instead of giving up (or possibly even punishing yourself, depending on how you feel about it) just try to think about what the next "good" thing is that you can do to improve your day over the last one.

Don't passively accept your failure to do what you wanted to do.

Using your junk food example, whenever you find yourself eating something unhealthy, stop and think about why you chose to eat it. Understand what made you fail and learn from it so you can prevent it in the future.

It takes a lot of will power to break habits. Once you break those habits it gets easier though but it's still a struggle. It's a real battle just to get over that hill though. Start with one thing at a time. Don't try too much change at once. It will be too much and you will most likely fail.

I'm about a year into my fresh start. It took a little fumbling and backsliding every once awhile but it literally took a year to get it all together.I run 15 miles a week. Lift 3 times a week. Don't eat fast food. Don't drink beer. Don't drink soda. Don't smoke. All those vices were things I had done for almost 13 years daily.

I didn't cut it all out at once. Took it slowly and one thing a time. I still drink beer once every couple of weeks. I'll eat out about the same interval but I don't eat fast food, just dine in stuff. I'll have a soda maybe once a month but I don't even want to finish it.

Anyway, take it slow. Don't expect to change over night or even in a week or a month. You can do it though.