Think of a snow-covered mountain. You're standing on top about to ski (or snowboard, whichever you prefer) to make your way down. Perhaps as a kid you sledded down wintry hills in your neighborhood. Over and over again. Did you always take the same routes down?
Our brain maps function in a similar fashion. If we keep taking the same route down the hill (i.e. think the same thought) we'll make deeper and deeper grooves. Eventually we'll become rigid and limited to those paths. If you've been thinking negatively for a long time, it'll take a lot of practice getting yourself out of those grooves.
But first of all, it takes awareness.
When was the last time you took a step back to observe your thoughts?
We're generally not aware of what is happening in our bodies, our minds, or our surroundings unless we start paying attention to it. For example, until I ask you how the ground feels under the soles of your feet, you probably weren't aware that indeed there is ground under your feet. Or maybe you're sitting down, so try shifting your awareness into how comfortable your seat is.
Same with negative thoughts. Until you start paying attention to the small moments where you're talking down to yourself, or calling yourself names, you won't be able to change them. Pay attention when your mind wanders, especially into the garden variety of "Can't believe I did that, I'm so stupid" or "this looks awful, I'm so fat". These moments are probably linked to your shame triggers.
Take a breath, and reframe. What else could explain that situation? Do a reality check. You're not stupid, maybe you were distracted from trying to do too many things at the same time. You may in fact carry a few extra pounds, but you can buy clothes in your size that fit and flatter your body type.
Most of all, you can choose what you think and how you talk to yourself. Putting yourself down is only feeding into the shame triggers (in these examples, intelligence and body image), and you'll be less inclined to change something about it. Believe in yourself and your ability to do better next time, and you'll be on the road to more positive thinking.
Mind your thoughts, as they become words. Mind your words, as they become actions. Mind your actions, as they become habits. Mind your habits, as they become your character.Mind your character, as it becomes your destiny.