There is a big difference between being mindful and being obsessive/negative.
Making mindful choices in line with your values? Wonderful. I can make progress by leaps and bounds doing that. Being an obsessive, guilt-tripping, stressed out, hard ass all the time? Fattening. Keeps me stuck.
That's how I look at it anyway.
Another telltale sign between mindfulness and obsession. Mindfulness happens in the present moment. "Right now I don't want any more." "I feel energized this morning so I'll do a longer workout." "Today I'll feel better if I don't eat the rest of the pint." Obsession tends to involve storytelling about the past or future. "I ate too much last night, so now..." "I'll never lose weight if I don't..." "Starting tomorrow I'm going to..." "From now on I'll just..." "I need to count calories because...(insert catastrophic future thoughts)."
Nearly all of the changes I made to the way I eat, I would do it just this once. Just this meal. If I like it, maybe I'll do it again. No pressure. Over time, I changed basically everything, but I did it without obsession and rule-making, and definitely without any guilt. If you enjoy the new habits, they'll stick.
I definitely do question why I feel a certain way, or why I chose something, or if a kooky thought is true. There's plenty to be learned there. But it's like few seconds of curiosity. It's not emotional. It's not agonizing. It doesn't go on for hours.