Sometimes, most times, all the time the hardest part of a project/task is just getting started. Why is inertia so hard to overcome?

Because when we become ensconced in habit, we get stuck.  It is like a rabbit who has just had a door opened from a cage it has grown to accept.

It takes, sometimes, the ingrained practice of doing the things we don't want to necessarily do, to affect a better outcome.

If I want to lose weight, I must change my food, my exercise and my outlook on eating.  Just wanting to be thin does not work.   So a goal must be achievable, and the skills needed to make it successful, focus, endurance and willingness to see it through are the tools needed. 

Once it is achieved, many impossible things are no longer impossible.  All it takes is knowledge of how to do it.  Otherwise we devolve.  There is nothing else for it.  You either change and grow or stagnate and die.