Why You Can't Understand

You understand written English just fine. You can follow slow, clear speech. But when a native speaker talks at full speed, it sounds like one long blur.

Connected speech is not sloppy. It is systematic. The same rules apply every time, which means you can learn them. Once you know how words melt together, the blur resolves into clear, predictable patterns. The fast English that once sounded impossible starts sounding normal.