So, I had the same sort of struggle a while back. And I have managed to fix it, recognizing what I had been doing wrong.

1. I did not know how to test different parts of my system. Going through other people' s testing example actually helped me a lot in this respect.
2. I did not separate the complex internal logic of my program, from its side-effects. For example put the mapping of the data in the same function as I was doing external requests. You learn how to manage problems like this by practice. But of course, be mindful of the side-effects of your functions. Avoid it when you can.
3. One other problem I had was that I wrote every test in the same manner. Tests that are not idempotent, and depend on external services, are useful, but you should use the facilities of your language to run them separately from your unit tests. In rust for example we use feature flags.

Testing and writing functions that can be tested are skills that need practice. For me, many of the patterns of the functional programming helped a lot. Since they insist on pure functions and pure functions are massively easier to test. Learning how people manage to write programs with pure functions may help you a lot, too.