Think of something you like doing or are already doing a little. If you use the stairs, make one extra trip. If you like to dance, put on one song. Make it fun and make it easy and make it tiny. You can expand later.
Working out can feel like just another thing on a long to-do list. But it is the part of the to-do list where you can focus on just yourself and your body. It is a way to care for yourself, and at the end of a workout I always feel better! More centered, I sleep better, and I have more energy the next day.
Set your goal and celebrate your success with each goal succeeded. Change that inner voice as you start and yes eventually that negative voice will be smaller and quieter, speaking up less often.