basically just do it.
You're thinking about it too much. Get yourself to do it without thinking about all the problems and conditions etc. It's like walking. If you consciously have to place each step, you'll never make it. So, remove your conscious part and let it flow.
For people with J preference, making a list and going through it helps. (left brain). ghantt charts, todo lists, action-items whatever
For people with P preference, making a story and going through it helps. (right brain). Mental movie, story board, mental dress rehearsal or whatever holistic ways of getting through it.
In all cases, reject thoughts that bring despair or worry or distraction. Basically all the things that are blocking your path and focus on the task at hand. When mentally preparing, always allow room for frustration and difficulties.