Starting anything new is incredibly hard.
It's frustrating, but after that initial beginning - it is actually natural to fall into a rythm.
You can see this starting difficulty at any time scale: from starting work in the morning or starting a new weekly exercise routine. The first minutes, the first day is always the hardest.
When creating new work this can also be the case.
That is why I start new pieces in the middle of the week.
I always leave pieces halfway done by the end of the day and finish the following day.
This came from advice from Ernest Hemingway:
The best way is always to stop when you are going good and when you know what will happen next. If you do that every day when you are writing a novel you will never be stuck. That is the most valuable thing I can tell you so try to remember it.
This means that when I begin in the morning I know exactly what I need to be doing - I don't have all the baggage of starting and warming up the engines.
Then by the time I do need to start I have so much momentum it is easy to begin
Try starting in the middle instead of at the beginning.