It is quite normal for humans to have negative thoughts and have moments of sadness. The real trouble is when these thoughts build up and dig into us and consume us. The can build anxiety or make our lives difficult by simulating moments of hopelessness without them actually happening. To read more about how bad these negative thoughts are and to find ways to tackle them, head on to the article below.
What are the negative thoughts? The Negative Habitat (buddingpsychologists.org)
This is a well-researched article and its nice to see how it gives a quick introduction of techniques used in psychotherapy as well. I remember how in Cognitive Behavioural therapy there were techniques such as Rational Emotive Therapy by Albert Ellis and Aaron Beck's work on cognitive distortions that are also an interesting read connected to negative thoughts. If there's anyone looking for more information on the topic, do read up on these interesting concepts as well!
This article on BuddingPsychologists titled Cognitive Behavioural therapy, also has some more research on the field, do check it out!