Before you start a task, your brain hasn't built a clear “map” of it yet, which makes the beginning feel uncertain and therefore harder. Once the first few minutes pass, that map takes shape and continuing gets noticeably easier.
That's why the reluctance before starting something isn't really about the task itself, it's about that uncertain threshold. Most of the time, the hardest part isn't the middle of the work, it's the very start.
A simple way around it is starting with the easiest possible step, without trying to make the first five minutes perfect. Once the map exists, the rest tends to flow on its own.