If so many experts and historians say the Bible is made up, full of errors, and has been changed, etc. then how can we trust what historians say about secular history?
I assume yours is more of a rhetorical question than an actual one. You seem to be making a point rather than asking a question. And, let me say, I think you are making a very good point. Generally, people hold the historical statements in the Bible to a MUCH higher standard of criticism than any of the other ancient historical documents. With most of “secular” history, statements seem to be taken as true unless proven false, but with the Bible, historians seem to take statements as false unless they are proven by non-Jewish, non-Christian sources. What Julius Caesar said is virtually gospel truth, but what Luke said is probably a complete fabrication. I am only slightly exaggerating here, and I assume this is the reason for your rhetorical question.
The answer to your question ought to come from an historian who is an expert in the analysis of sources, and I am not an historian. Let me just make a bit of an amateur comment. I do believe that we can learn real information from ancient writings. For example, Alexander the Great was a real person, he did conquer a massive amount of territory and defeat the Persians, and he did marry Roxanne, a Bactrian princess. These can be taken as “facts.” However, all ancient writings should be taken in their context, and their validity should be analyzed based on other sources and a reasonable analysis of the likelihood that the particular author is likely to be more or less biased. By this standard, the histories of the Assyrian kings would be highly biased and a source of some but only limited truth as they are extremely biased, and include much that is mythological. By the same standard, the histories of Thucydides or Tacitus would have a lot of truth, but should still be looked through the lens of the bias of the authors. By this standard, the Old Testament histories such as Samuel, Kings and Chronicles would be perhaps at the level of a Tacitus or a Thucydides—generally quite reliable, but should be read through the lens of a particular bias. There is a good deal of bias in Chronicles, for example.
My opinion is that, by this standard, the New Testament authors would be of the highest level of reliability for several reasons. The wrote within a generation or, at the most, two generations of the events, and they wrote to a community that included many who were eye-witnesses of the original events, and a which already had an oral history which could be a check on any misinformation or exaggerations. I believe that the gospels and Acts are of the highest level of reliability of ancient histories. But, bear in mind that I am an amateur when it comes to history!