I've changed my diet many times in my life. At the age of 10, I abruptly stopped eating meat. In my 20s, I switched over to a mainly organic diet and started cutting out most junk food and fast food. In my 30s, I gradually went vegan. Every time I made a change to my diet, I used different approaches and techniques. Some were better than others. But I didn't "go on a diet." Instead, I changed my eating habits. I've been able to sustain all these changes over the long term, without feeling like I'm missing out on anything. This is how I did it (and how you can too!)
Note: I personally am vegan and eat mostly organic foods, but these methods can be applied to any changes you want to make to your diet, whether it's cutting out sugar, eating whole foods, or any other diet change you want to make. I'm not here to tell you what you should or shouldn't be eating, because that's up to you!