It sounds like you need to learn how to become mindful of your thoughts.
You think that "no big difference is going on in my life" but that is simply not true. You do not live in a hermetically sealed bubble with zero outside influence, where your days and nights are exactly the same 24/7 - am I right? You live in the extremely complex and kinda overwhelming world of the 21st century where you are constantly bombarded with outside information, experiences, and influences.
Right now you are oblivious to how your brain is interpreting that mass of information and interactions, and you are oblivious to the little stresses that your thoughts are creating.
Our nervous systems were designed to react to stress. Tens of thousands of years ago in the primitive world, the stresses were few and far between, and when they happened they were big scary stresses, which were literally life-threatening. This is the environment our brains were designed for, and they have not evolved past that environment, which means that our stress response is not up to the task of living in today's world.
This is why we talk about mindfulness all of the time. As to how you learn mindfulness, that's a huge topic with many different avenues. Tell us what you think about what I've just said here, and maybe we can all come up with some suggestions.