Break down to smaller goals and ACT. There is no better motivation and there isn't a better technique than this. It helps you to focus on one small thing and you forget about the rest of heavy stuff that you think you never finish.
Most importantly: If you break a goal in minigoals and finish all those minigoals(ultimately finishing the goal), you can compare your old doubts with your new reality. Those doubts were false, it was just your head playing games on you, and you tricked it. This reinforces your inner voice and when new doubts arise you can more easily hush them away ("hey, Ill just break this down like I did last time and finish it like I finished before, did it once, will do it again and you just shut up").
I doubt now and there EVERY DAY but I already developed those techniques so I can keep on marching. Otherwise I would get nowhere.