Your thoughts.
The number one thing holding you back is the thoughts you have regarding the thing you want to do. One thing that has helped me in the quest of doing things more often is this thought:
Don’t trust all of your thoughts.
That’s it. You don’t need to listen to each and every thought you have. They’re just not that productive or valuable. You have a choice to not listen to them.
Not govern them. Not to steer them. Just not to listen to them at that moment.