When you don’t think you can accomplish your goal, it’s because you’re looking too far down the road.
You’re thinking of all the things that could go wrong and of all the obstacles you must overcome, none of which exist if you can focus on the immediate.
When you’re overwhelmed and you’re afraid you won’t be able to achieve your goal, focus on what you can do right this minute.
Make it small enough that you’ll actually do it.
Revel in the success you feel.
That will give you the motivation to stay on your path.