What's the best way I can stay on track with my diet/workout plan?

I started a diet ( nothing drastic) but I'm watching what I eat and drinking lots less of sweet drinks and replacing those with water. But it is so freaking hard to pass up the stuff I used to get. I was going to get a bikini and hang it up and let that be my encouragement to loose the weight but that wouldn't be smart since I'm wanting to loose thongs would change. I'm struggling to stay on track. I'm about 155 and am wanting to loose at least 20 pounds by June. Does that sound realistic and im about 5'4 .
What's the best way I can stay on track with my diet/workout plan?
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